Karthus Build Guide by Urza
Not Updated For Current Season
This guide has not yet been updated for the current season. Please keep this in mind while reading. You can see the most recently updated guides on the browse guides page.
Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
First of all I want to say that english is not my native language, so please, forgive my mistakes :o(
Why I decided to write a guide right for Karthus? Well, I love fantasy games and I always play mages - regardless what's their role in the game. When I finished the tutorial game (as Ryze, of course) and started Co-op vs. AI where I had 90 seconds for pick a hero but I didn't know any; so I picked the one who looked as mage the most. When the game started, I saw that my choice was great - Karthus was the necromancer, pure mage.... exactly what I wanted to play. Since that time I was learning to play many other chars (some of them I have handled quite well, some ruined all my effords): AP carries ( Anivia, Swain, LeBlanc, Morgana Brand, Malzahar, Veigar, Ryze, Orianna, Xerath, Annie), AP junglers ( Fiddlesticks, Cho'Gath, Maokai), supports ( Zilean, Lux, Nidalee, Janna, Sona, Soraka), AP tanks ( Cho'Gath, Maokai, Galio, Singed, Leona, Amumu), AP geeks ( Heimerdinger) or even AD carris ( Vayne, Sivir, Ashe, Caitlyn) but I have never found any hero who suits me as well as Karthus. He's just my best!
Although this is my first guide, it will be quite alternative - i.e. you can find here some things that other guides or good players don't recommend but even if many of those things independently really don't comply with Karthus, I think they can work together very well; I am not pro or excelent player but I have played as Karthus more than 500 games (win ratio 59.9%, average score 9.1 / 4.5 / 9.2 in 152 ranked games - watch screenshots) with many different builds and this is the one I like the most.
I decided to write this guide because I hope it helps somebody and I am also curious for your opinions and potential discussion under this guide; so vote and comment, please. But I ask you - before you write something like "KARTHUS WITHOUT ARCHANGEL'S STAFF IS SO N00B!!", read whole guide and consider my reasons why I play Karthus this way. In case of your downvoting, please, try to play that and write me the reason (not just "THIS BUILD SUX CAUSE JUST DUMBASSES BUY ROD OF AGES!!").
PS: As my playstyle ripes, I am changing this build according to it. I mark every change by keyword edit or added with proper date. If you are interested in my changes, just use your text-searching function of your browser (ctrl+f) and find words edit and added.
Pros & Cons
Why you will love to play Karthus:
- awesome damage dealer
- all abilities can hit more than one target
- definitely the best passive in whole game - Death Defied
- spamming ability with one second cooldown - Lay Waste
- debuff and cc in one spell - Wall of Pain
- great dps around - Defile
- everybody will hate you due to your ulti - Requiem
- you can make enemy angry; instead of playing he writes about noobs/OP chars - so you can kill him again
- he is the king of teamfights
- even if squishy mage, he can initiate (in some specific cases)
- insane farmer
- restores mana by farming
- long spells' range
- specific playstyle
- his joke is funny
- he is undead
Some imperfections you might notice at divine Karthus:
- with full build is weaker than many carries with their full builds.... but he can own them many times before
- quite slow.... but his ulti can damage enemies wherever
- item dependent.... but he can afford that xD
- bad escaping ability when Wall of Pain on cd.... but good otherwise
- very squishy.... but which mage is not? (Forget Cho'Gath etc. - I mean TRUE PURE mage xD)
- focused in teamfights.... but fights even with activated Zhonya's Hourglass AND kills after his own death
- many chars and items can counter his ulti.... but most of them can't xD
- it's hard to master.... but easy to play quite good
Spells (some call it skills but mage's skills are spells for me xD)
Passive: Death Defied
Description: When Karthus dies, he becomes ghost who can cast spells (but can't move) for 7 seconds. In this form Karthus can't be target of spells or abilities and is completely imune to attacks and/or AOEs (so his spells can't be interrupted). Spells casted in ghost form cost no mana.
As I mentioned, I consider Death Defied as the best passive in game. Why? There are many ways how to use it!
- If you need to be positively and absolutely sure that anyone can't interrupt the channel of your Requiem, cast it as a ghost. When common teamfight starts, you want to use your Requiem as soon as possible but you can't due to danger of interrupts. If you die there, it's easy to cast Requiem now.
- Bigger skirmish (e.g. 3v3, 3v4, 4v4, ....) just have finished. Two or more enemies with almost no hp disappeared away. You are out of mana (potential Clarity on cd) or there is no safe place around (e.g. not warded enemy jungle) and you feel you will get ambushed. You can kill many by Requiem but you don't have 8 seconds for recall and 3 more seconds for Requiem's channel. You can commit suicide (e.g. under enemy turret) and cast Requiem as a ghost. You have to be -of course- absolutely sure that your kills will worth it and your suicide will really save your time.
- 1 vs 1 combat against an equal foe is not you-or-him; it's you-or-nobody. If your health goes down as fast as his, you aren't risking too much when you continue fighting. You win? Good for you. You lose? Not so bad, turn on your Defile (cost no mana) and Lay Waste him. You both are dead now.
- You can initiate in some cases (e.g. you are on low hp and you know that you won't survive anyway). Turn on your Defile, Flash-in just right to the center of enemy group, turn on Zhonya's Hourglass (this will cause chaos in enemy lines - they all will try to focus you but it won't be possible for 2 seconds). After two seconds you can be quite sure that your ghost will stay in the middle of following teamfight (unless your team is sleeping or surprised and confused by your action - in this case you will be treated as idiot and you won't be able to explain that this could be a good idea).
Q: Lay Waste
Description: Spell is targeted to a small area in quite long range. It damages all enemy units in affected area. If there is just one target, Lay Waste deals double damage.
This is probably the best spamming spell in the game (cooldown 1 second). Although it seems like "burst", it's dps in fact (cause of so low cd). Proper using of this spell differs Karthus-the-master and Karthus-average-player. Due to 0.5sec delay, you have to predict enemy movement to get hits. How to learn this? Practice, practice, practice, practice xD. And one old good trick - throw Lay Wastes behind your enemy; when he notices that you are attacking, he might instinctively move backwards and get hit.
Very important aspect of proper using Lay Wastes is to get advantage of double damage for single target. When you are killing a champion who is staying next to some minions, try to throw your Lay Waste out of minions (if it is possible).
Lay Waste is also very good spell for last-hitting creeps. If you are super-pro player, you can probably lasthit every minion by autoattack; otherwise there are four important advantages of last-Wasting: bigger damage, aoe damage (to more creeps in the same time), longer range, better prediction (this is probably very subjective but I estimate Lay Wastes delay much better than autoattacks).
You can use your Lay Waste as sight ward (but very small one). If you Lay Waste bushes, you will see very small area for very short period of time. That's good in case of many enemies in one small bush - not for one enemy in wide bushes because you probably miss him (and if you miss, you won't see - because you see just the small "impact" area).
W: Wall of Pain
Description: Karthus conjures magic wall (the higher skill level the wider wall) for 7 seconds. Enemy magic resistance and movement speed is reduced for 5 seconds when he pass through the wall. Approximately 14secs cooldown (with some CDR - so basic cd is something about 17secs, I don't know exactly).
This is an excelent crowd controll and debuff spell. There are many ways how to use Wall of Pain:
- Put the wall across the teamfight area to reduce MR to as many enemies as you can. The slow is quite good side-effect.
- While chasing enemy, put the wall in front of your target.
- While chased by enemy, put the wall between you and chasers (since you are not Anivia, you can pass your wall - if they are close to you, put the wall right on you).
- Many enemies (naturally) try to don't pass the wall. You can use it in jungle - if you want an enemy to go some way, "block" another way by the wall.
- You can see your teammate chased by many enemies. If you can't face them, you can help your teammate without danger of get stunned and killed. Just approach them and put the wall in front of your teammate and chasers. Because the wall stays there long enough to your teammate pass through and the enemies will get slowed, you needn't even to get close.
- Your team is trying to push the turret and enemies are standing below. Put the wall directly on the turret (it will come through). Your enemies are moving slowly around their tower and they might get MR reduced.
- Your enemy is going to push your turret. Put the wall into their way (e.g. at stairs to base). They will come to you with reduced MR (and if you hurt them a lot, they will run away slowly).
- You can use Wall of Pain as sight ward. If you put it into some bushes, you can see enemy within. If you are attacking a turret, you can put the wall to the side from you are expecting enemies - you will see them earlier.
Description: Toggle-type spell. When activated, it damages every enemy unit around Karthus (very nice range) each second and cost him quite lot of mana. When deactivated, it restores mana every time Karthus kills an unit. Area of effect is moving with it's caster - he is allways in the middle.
This is great AOE. I am temped to write that it's the best AOE in the game but you wouldn't believe me that one char has the best passive, spamming spell, AOE etc. xD. Anyway, if this is not the best AOE, it's one of the best.
The only disadvantage of Defile is it's mana cost in combination with tempting of unskilled or unexperienced summoners to turn Defile on very often and/or let it turned on for a long time. It might cost you all your mana supplies in a short moment. To deal with it, I follow two simple rules: do NOT turn your Defile on till you are level 8 and do NOT turn your Defile on just for creeps killing till level 11 - 13. There are two exceptions to both rules: you are dead (so it doesn't cost mana) OR you have a lot of mana and you know you will go base anyway (e.g. you are on very low hp). One fail you can do very easily (I did many times when I was learning to play Karthus): you stay somewhere (e.g. mid-lane), one foe attacks you and you feel you can defeat him. You turn on your Defile instinctively and start Lay Wasteing him. You are watching his health and your health to estimate who will live and who won't. Suddenly - you are out of mana. This doesn't mean that you shouldn't turn your Defile on (it is that case you know you will go base anyway) - you just should watch your mana!
This spell is very important in early-game but just for it's passive - it gives you mana from every killed enemy unit which makes you able to hold your lane for very long time without mana regen. In mid-game you can kill enemy champs by this (and of course you still use it as mana source). In late-game you can kill & farm with your Defile (but realize that if you kill creep by this, you will get no mana).
Killing with your Defile is very easy. Just chase your enemy (or stay in teamfight) with Defile turned on. There is just one catch: it is definitely NOT your best way how to kill in 1v1 fight where one well placed Lay Waste damages as much as two or three seconds of Defile (and cost you four times or five times less mana). So the difference between good and bad Karthus is that the first one will chase his enemy with Defile AND shoot him by his Lay Wastes but the second one will just chase with Defile (and run out of mana).
Farming with your Defile is even easier than killing. In late-game you needn't to stop your movement due to farming - just go where you are going and when you are passing by creep wave, turn on your Defile. If you have enough AP, you needn't even stop for a second. There is one handy trick for Defile-farming: go into creep wave, turn on your Defile till those creeps won't be almost dead and Lay Waste them. It restores you some mana (if you kill them by Defile, you get nothing).
Dying with your Defile is a good rule. Everytime and everywhere you die, turn on your Defile then. Even if it looks needless. You never know who will walk around xD.
Shooting blindly with Defile is also a good way how to use this spell (if you have enough mana). Although Defile doesn't ward (even if you are damaging some enemies in bushes, you can't see them), it damages all enemies in range (regardless of their visibility). You can kill invisible fleeing enemies on low hp (or Teemo standing somewhere). Tip: your Defile's range is wider than Akali's Twilight Shroud so she can suffer even if you can't see her (but consider that this is quite bad idea when she's on full health and your team isn't around - she hurts you more than you can hurt her without your Lay Wastes).
Description: After three seconds channel, this spell damages every living enemy champion in any distance. If channel is interrupted, spell is canceled and you have to wait till next cooldown before casting. Cooldown 180s / 150s / 120s (in late-game you can cast it approximately each 90 seconds due to CDR).
This spell is the main source of enemies' hate xD. If you kill an enemy by Requiem, he might get angry, call you n00b, your ulti noobish, your char OP.... this is very good because who is writting instead of playing becomes easy to kill.
In early-game and mid-game use your ulti every time you can kill somebody that noone else can. Try to not steal! Even if this is quite hard to decide what's steal sometimes. There is one exception: enemy (without hps) runs to his turret (or some other dangerous territory) and your teammate must dive to get him - in this case I prefer stealing over possible feeding. And there is one exception in this exception (xD): If my teammate is quarrelsome or touchy, I rather let him dive (I think one free kill for enemy is better than flame in team-chat).
In late-game try to save your Requiem for teamfights. You can initiate or finish by Requiem (both possibilities have advantages and disadvantages). Since you can't channel close to your enemies, initiating by your ulti means you miss the teamfight's start. On the other hand you can deal full damage to every enemy champ if you cast Requiem before teamfight starts. In most cases I prefer jump into teamfight, deal as much damage as I can, die there and cast Requiem in ghost-form ( Death Defied). I initialize teamfights by Requiem when I'm too far to get there in time or I'm quite sure that enemy won't interrupt my channel.
You should check the map all the time to use this spell proper. In early-game and mid-game I recommend to write into team-chat every time your ulti is ready. Despite that keep checking the map! You should also have enough mana to cast Requiem all the time.
If you see an enemy on low health, you should know if you can kill him. See his health and MR (it's good to remember their builds - not in every case you have enough time to watch it). The formula for damage calculation is: true damage = magic damage * (1 - magic resistance / (100 + magic resistance)). You know their resistance as well as your Requiem's magic damage. For faster calculation of that formula evaluate the second part first (MR / (100 + MR), you can remenber some constant results: MR(0) = 0, MR(30) = 0.23, MR(50) = 0.33, MR(75) = 0.43, MR(100) = 0.5, MR(150) = 0.6, MR(200) = 0.67, MR(300) = 0.75).
Even more important than true damage calculation is to know champions and items which can annihilate (or minimalize) your Requiem's effect. Some examples (the list is not full): Anivia's Rebirth, Morgana's Black Shield, Orianna's Command: Protect, Zilean's Chronoshift, Sona's Aria of Perseverance, Soraka's Wish and Astral Blessing, Janna's Monsoon and Eye Of The Storm, Kayle's Intervention and Divine Blessing, Lux's Prismatic Barrier, Vladimir's Sanguine Pool, Sivir's Spell Shield, Nidalee's Primal Surge / Swipe, Taric's Imbue, Alistar's Triumphant Roar, Shen's Stand United, Blitzcrank's Mana Barrier, Dr. Mundo's Sadism, Nocturne's Shroud of Darkness, Tryndamere's Undying Rage, Zhonya's Hourglass, Banshee's Veil, Hexdrinker, ....
If your enemy disapears and you don't know if your Requiem can kill him (e.g. his saving-ability might be ready/on cooldown OR he have high/low hp), decide by actual game situation. In defensive games with lower scores I recommend to cast (every kill is valuable AND there is a little chance for repetition of the same situation till the next cooldown). In offensive games with high scores I recommend not to cast (the kill is not so valuable AND there is quite good chance for less risky -or multi- kill in a short time).
And one more important thing in conclusion: Death interrupts your channel despite your Death Defied. If you die while channeling, the spell is interrupted even if you are still able to cast.
Summoner Spells (sorted from the best to the worst - for Karthus)
Flash is ultimate and superior spell. You can use if for offense as well as for defense. I can barely imagine an interesting game situation where Flash wouldn't be useful.
Some Flash-offensive opportunities for Karthus: flash into teamfight with Defile turned on, flash under turret and kill there (or flash away after that), flash closer to running enemy tu put Wall of Pain in front of him, flash across the teamfight battlefield to focus better targets.
Defensive flashing is the same for every champ: flash over some terrain barrier when you are chased, flash out of enemy range, flash to your turret when you are ganked, flash out from aoe dps spells.... but every time I use Flash for defense, I regret that potential missed offensive Flash - necromancers should kill, not run xD
I think Flash is a need for every predator - not just for Karthus.
This spell is really great because there are many ways how to use it:
- Ganking. Especially if enemy chases your teammates behind your own minions - you can teleport behind their backs. Then just kill xD
- Fast returning to your lane from shopping, health regenerating, mana regenerating or ganking other lane.
- Pushing. You can join (from any place) your pushing teammates. Or you can backdoor without your teammates.
- Defending. Less "undead" usage at the first sight.... but you can kill pusher while you jump there xD
Edit (16.11. 2011): I use Teleport against some hard enemies (like Kassadin or Kennen) because I probably can't defeat them even by hard aggressivity which I can afford because of Clarity OR sometimes I use Teleport when I think that my jungler can provide me well by the blue buff since early game till late game.
Edit (22.11. 2011): I use Teleport instead of Clarity on higher ELO because against good mid lane foes there is quite low chance to beat him in early game due to aggressivity which you might affort because of Clarity.
Clarity is quite controversial spell. Almost every time I pick this spell, somebody of my team writes something like "clarity n00b" or "Karthus with clarity = gg". Despite that I consider Clarity as very good spell.
The strongest argument against Clarity is: "Karthus don't need it." Yes, that's true. Due to Defile's passive Karthus can be well played without Clarity. As well as he can be played without e.g. Ignite. The point is that you can be much more aggressive with Clarity. Your Lay Waste spamming become insane (it's very good even without it but it's definitely better to throw more Lay Wastes than less Lay Wastes).
The second argument against Clarity is: "That spell is useless in late-game." Yes, it's not as useful as in early-game but it's not useless. Possibility to get some mana is handy very often. Furthermore, even if 805 mana is not so much for 18 level Karthus, 402 mana can be enough for many ADs in your team.
Moreover you can meet whatever on your lane. If you play against better player than you are, Ignite doesn't help you, Teleport helps a little, Clarity can save you - even if he is better and you can't kill him, you can hold him away by Lay Waste spamming. Sometimes you can also meet very strange beasts on your lane: e.g. Mordekaiser - with Clarity you can crush down his shield by Lay Wastes brute force (you need to do it repeatedly because every time you take him some hps; then you will need Wall of Pain and/or Requiem and/or few seconds of Defile). With a lot of mana you can hold the tower against more than one enemy.
Edit (22.11. 2011): I use Teleport instead of Clarity on higher ELO because against good mid lane foes there is quite low chance to beat him in early game due to aggressivity which you might affort because of Clarity.
Rating of this spell depends on what you are planning to play with Karthus. If you are going to jungle (and Karthus really can jungle well), Smite is a must. Otherwise you don't need this.
Since Karthus is quite slow hero Ghost can become handy. But I don't recommend to pick it as summoner spell because you can substitute Ghost by your Wall of Pain very often (slowing your enemies mostly results in the same as accelerating yourself). Of course, it might happens that you will need Ghost for something that you can't handle by Wall of Pain but it not worth summoner spell slot.
(added 18.11. 2011)
Surge could help you in your offense when you are killing your lanning foe. It can be also used in teamfight for more damage; it helps you probably more than Ignite. Although Surge is not as handy as some other spells, it is definitely not bad choice.
Good offensive spell but it's better for burst damagers who fire out their burst and can't kill due to cooldowns. Karthus is (great) dps damage dealer so if you want to finish somebody off, you just stay there for longer time. If he escapes on low health, you can use your Requiem (and if it isn't enough anyway, Ignite wouldn't probably change it). Healing reduction is very good aspect of this spell but it doesn't worth to pick it.
It's good spell for the first blood but you are solo-laner in most cases (that means you probably don't get the first blood anyway). Furthermore, I consider Heal as quite defensive spell so I don't like it because it doesn't fit my playstyle.
Since Karthus is squishy hero and has no excelent escaping ability (like e.g. Kassadin), Cleanse won't help him too much. If you get focused, you die very fast regardless crowd controls. Quite good usage of this spell is to cast it when you get focused and you can't activate Zhonya's Hourglass due to e.g. stun.
Very good spell - but not for Karthus. Of course, you can use it well, but it's much better to write "cv" into team-chat and ping the map.
Slowing effect of Exhaust can be substituted by your Wall of Pain. So you can decrease enemy's attack damage but that's not your role - you are killing predator, not support.
(added 18.11. 2011)
Since Promoted can be casted on cannon minions only, it can not be used by all five champions for great push which makes Promote just help-pushing spell. I think that picking this spell is wasting for Karthus (he is killer, not pusher).
This is good summoner spell but you aren't the one who should have it. Your spells should support rather your own abilities than turrets.
Edit (18.11. 2011): Fortify was removed from League.
I have heart about good use of this spell once. Someone allegedly combined Teleport with Revive to participate in one teamfight twice. But I consider Revive as useless spell due to it's cooldown. As wise summoner said: "Revive is not the reason why you come back to play faster but the reason why you die."
I have never ever play AD Karthus so I have never picked Rally. If you try it, tell me the result xD
Edit (18.11. 2011): Rally was removed from League.
I usually start with Doran's Ring OR Boots of Speed and 3x Health Potion. Then I upgrade Boots of Speed to Sorcerer's Shoes and I buy some more Health Potions if my laning opponent is harassing me too much. Afterwards I very often buy fast Mejai's Soulstealer followed by Blasting Wand or Sapphire Crystal or Ruby Crystal (or straight Catalyst the Protector) - the purpose is to build Rod of Ages but the order of buying it's parts I choose according to actual game situation ( Blasting Wand for offense; Ruby Crystal or Catalyst the Protector for defense). After Rod of Ages I build Rabadon's Deathcap followed by Zhonya's Hourglass. Then I buy Will of the Ancients and I finish by another Rabadon's Deathcap or Rylai's Crystal Scepter or Void Staff. When I get completely full build I start to drink Elixir of Brilliance and Elixir of Fortitude, maybe Elixir of Agility (and Oracle's Elixir if needed). If the game still doesn't finish, I consider about selling Sorcerer's Shoes to get space for Void Staff or Rylai's Crystal Scepter or Rabadon's Deathcap (that depends on the item bought before elixirs).
(added 16.11. 2011)
I start with Doran's Ring if I pick Teleport because of mana regen. Futhermore I don't need to buy Health Potions because I am able to fast jump to the base and back by Teleport (then I usually bring some Health Potions). The main problem of this start is movement speed. If you start by Doran's Ring without Boots of Speed, be careful of faster enemies with Boots of Speed on your lane.... moreover, you are disadvantaged while attacking because they can easily run out of your range.
And one last thing: if you follow this evil build EXACTLY (with Doran's Ring), you get 666 hp at level 1 which makes this guide good for any Necromancer xD
I start with Boots of Speed if I pick Clarity because Karthus is quite slow hero. I think it's good to improve his movement speed for some early-game chasing in jungle or emergency movement somewhere etc. Furthermore, this item is quite cheap so I can buy three Health Potion (which I am using very often - you can read more in the next paragraph). Another advantage of Boots of Speed is quite cheap upgrade to Sorcerer's Shoes.
I like Health Potions. And I like starting with three of them (if possible) especially if my laning oponent hasn't any (because he bought e.g. Doran's Ring or Doran's Shield). Why? Because I will gladly accept every opportunity to harass then - even if I get some damage from my opponent as well because my health can be restored, my opponent's health can't if he hasn't ability to heal himself (like e.g. Morgana) naturally. If he runs out of health I take place between him and fighting creeps and I start Lay Waste him. He has to stay out of exps and gold or return to the base which costs him more exps and gold as well (or he can call jungler to gank - so it's good to play this with one finger on Flash key).
Very good item for every mage despite it's low cost (you can upgrade Boots of Speed to Sorcerer's Shoes only for 750 gold). Mages (including Karthus) are quite slow which makes map movement quite hard for them; because of Sorcerer's Shoes you are able to move faster. Forthermore, Sorcerer's Shoes helps you to get through enemies' magical resistances which makes this item nicely offensive (thus suitable for your build).
Well.... I think this item is as controversial as potential Clarity (if you choose to pick it). Sometimes, after I build it, my opponents -or teammates- tell me something like "You don't know nothing about AP building." Most mages build here Rabadon's Deathcap or Archangel's Staff.
Why I prefer Rod of Ages over Rabadon's Deathcap or Archangel's Staff? Well, Karthus is dps champion - not burst. When I play LeBlanc, Brand or Veigar (or even mixed dps/burst like Anivia) I definitely build Rabadon's Deathcap (or Archangel's Staff if I play mana-wasteful champion like Brand AND I assume I won't get blue buff - e.g. due to AP jungler like Fiddlesticks who needs it). Since Karthus is dps hero, he deals as much damage as long he is able to stay in combat.
So let's compare Rabadon's Deathcap with Rod of Ages (and remember - Rabadon's Deathcap is almos 1.2x more expensive so Rod of Ages is handicaped in this comparsion). Level 10 Karthus has about 201 AP and 1140 HP with Rabadon's Deathcap OR 60 AP and 1590 HP with Rod of Ages. So he deals 360 damage per second by Lay Waste and 120 damage per second by Defile (summary 480 damage per second to single target) with Rabadon's Deathcap OR 280 damage per second by Lay Waste and 85 damage per second by Defile (summary 365 damage per second to single target). What that means? Rabadon's Deathcap gives you 32% more damage than Rod of Ages but Rod of Ages gives you 39% more time in combat. I know that it's much better to deal damage in short time but if enemy is burst, that 450 HP can make the difference between your instant death and his staying there on cooldown and receiving your damage; furthermore, Rod of Ages gives you more mana and I made all calculations with completely discharged Rod of Ages and I compared 3035 cost item with 3600 cost item. Now, let's assume charged Rod of Ages and one more Blasting Wand bought (3600 gold for Rabadon's Deathcap and 3895 gold for Rod of Ages and Blasting Wand - the cost difference is even less than difference between Rabadon's Deathcap and Rod of Ages). Karthus still deals 480 damage per second with Rabadon's Deathcap but 412 damage per second with Rod of Ages and Blasting Wand. Now it's 16% more damage with vs. 55% more time in combat (notwithstanding 725 more mana). I appreciate that this way of items comparsion is strange and quite misleading - in late-game teamfights you die almost immediately after you get focused regardless few more health; but in early-game fights is your health much more important.
Another reason for building Rod of Ages instead of Rabadon's Deathcap is higher variability of it's subitems. In case of bulding Rabadon's Deathcap you will just buy Blasting Wand first and Needlessly Large Rod (btw, cool item's name - isn't it?) then. But when you are building Rod of Ages, you can freely decide to start with Blasting Wand for offense or Ruby Crystal and Catalyst the Protector for defense. Furthermore, Rod of Ages' subitems are cheaper so you are more free in decisions when return to your base for shopping (you can use your gold better to improve your build more smoothly by items for 400, 450, 475, 850 and 860 gold than by items for 860, 1140 and 1600 gold - especially in early-game).
Great offensive item. The best one. You just have to get it. It has the highest flat AP bonus among all items and overmore it bonuses your AP by 30% ( Rabadon's Deathcap bonuses itself). It's only disadvantage is it's cost (and it's subitems are also expensive) - this is the only reason why I build Rabadon's Deathcap as my third item (with Karthus; with some burst mages I build it as second).
Although it looks like quite defensive item, you can use it for offense very well. You can turn on Defile and activate Zhonya's Hourglass then; your Defile still damages enemies which is great in teamfights. Standard way is just use it when you're focused; you stay alive two seconds more in the worst case (but your enemies can change their forcus in that two seconds). Alternative you can use Zhonya's Hourglass for initiate. Mostly it's not wise to initiate but there are some exceptions (enemies are on low health, you are on low health and you want to die in the middle of teamfight because of Death Defied, enemies are squishy, your team is squishy, ....). If you want to initiate, you have to be absolutely positively sure that your team will follow IMMEDIATELY (if not, your death will be futile) AND your enemy won't run away (e.g. they are defending something) AND your Flash is ready as well as Zhonya's Hourglass. Then turn on your Defile, use Flash to reach ideal position (i.e. all your enemies are within your Defile), cast your Wall of Pain to make their escape harder and activate Zhonya's Hourglass before you get silenced or stunned!! You caused chaos in enemy lines (they tried to focuss you but they failed) and your teammates are attacking now. If they aren't pro players, there is quite good chance for their mistake in focus (if they want to focus you, they have to wait two seconds and it's quite possible that they fire at you much more damage than they need; otherwise they focus your team and you may survive for a few seconds more). You die there -of course- but you are still fighting because of your Death Defied. But remember that this is about teamfight initiating - NOT about reckless 1 vs. X feeding (although you are great killer, there is no way for you to succeed in this action alone unless your enemy are on VERY low health).
Zhonya's Hourglass has quite short cooldown; you can use it almost in every late-game teamfight or bigger skirmish. But be careful - Zhonya's Hourglass can't be activated as long as you're silenced, suppressed, stunned, feared, taunted or knocked up.
This item gives you some AP (50 + 30 = 80) and 25% spellvamp. It also has aura which gives 30 AP and 25% spellvamp to surrounding allies. It's great item for dps mages - especially for those with aoe spells. This item can give you some more time for staying in teamfight which means higher damage to your enemies. You reach this item quite late and it's possible that some of your teammates has already built it and you receive bonus from it's aura anyway (but it's unique - so it doesn't stack). In this case consider building of another item like Void Staff, Rylai's Crystal Scepter or another Rabadon's Deathcap (I know, Rabadon's Deathcap doesn't stack but I think you could have more than one - I will explain later).
If the game lasts long enough for me to take sixth item, I choose among second Rabadon's Deathcap, Void Staff and Rylai's Crystal Scepter.
Rabadon's Deathcap: I know, I know, I know, I know - Rabadon's Deathcaps don't stack. Despite that I think it could be good idea to build another one. Even if +30% bonus is unique and doesn't stack, the flat +155 bonus -of course- does and moreover it's bonused by +30% from your second Rabadon's Deathcap which makes it +201 bonus in the end. There is no other item in the game that can give you so many AP - that's the reason why it's not bad idea to build Rabadon's Deathcap although it doesn't stack. Another advantage of having two Rabadon's Deathcaps is that your enemies may write into chat something like "n00b Karthus Rabadons doesnt stack HAHAHAHA" which is good as long as many of them is spamming instead of playing (taunt them to more writting if possible - but don't be an idiot and don't flame when you are alive). Unfortunately, there is also very similar disadvantage - even your teammates can spam your team-chat by something like "n00b Karthus Rabadons doesnt stack HAHAHAHA". In this case just don't react if you don't want to risc a flame.
Void Staff: Build this item if you fight against enemies with high magic resist. By "enemies with high magic resist" I don't mean their tank with low damage output (you don't focus him anyway). Build Void Staff if there are many enemies with magic resist OR if there is just one which damages your team a lot. Frequently asked question is: "What's better - AP or MP?" You can sometimes find answer like: "I was calculating it very for very long time and the result is that if your target's magic resist is greater than 100, MR is better than AP, otherwise AP is better than MR." That's ridiculous. That calculation is very easy at the first and secondly the result depends on mage's AP, mage's MP, spell's AP ratio and spell's basic damage. Let's calculate it for 18 level Karthus with this build. Karthus' flat MP is 28.5 and AP is 622 with Void Staff or 733 with another Rabadon's Deathcap. Lay Waste's basic damage is 120 and AP ratio is 0.3 - i.e. 306 magic damage (with 28.5 flat and 40% percentage MP) from Void Staff OR 339 magic damage (with 28.5 flat MP) from Rabadon's Deathcap. So we have this equation: 306 * (1 - ((MR - 28.5) * 0.6) / (100 + ((MR - 28.5) * 0.6))) = 339 * (1 - (MR - 28.5) / (100 + (MR - 28.5))); solve it and get MR = 61 for Lay Waste. If you solve the same equation for your other spells, you get MR = 60 for your Defile and MR = 49 for Requiem. For higher MR you deal more damage with Void Staff, for lower MR you deal more damage with Rabadon's Deathcap.
Rylai's Crystal Scepter: I usually don't buy this item. 500 health bonus is not so advantageous in so late-game. Since you are so squishy hero without any armor or magic resist, there isn't important difference between 2470 and 2970 health (yes, it's 20% more time to die since you become focused but in late-game's teamfights is that basic time damnedly short - so 20% is -zounds- 5x shorter). 80 AP is good -of course- but it isn't interesting when compared with Rabadon's Deathcap or Void Staff. And -finally- slow. Yes, slow is good. Because Karthus has multi-target spells only, Rylai's Crystal Scepter's slow is just 15% for him. That's not bad but if you can hit enemy by your Defile or Lay Waste, you can slow him by your Wall of Pain as well. So the question is: How often do you need 15% slow for enemies that you hit by Requiem OR how often do you need to slow enemy that you can reach by your Defile or Lay Waste while your Wall of Pain is on cooldown? The answer is: rarely.
When your build is finished and you still have some gold, buy Elixir of Brilliance (extreme useful), Elixir of Fortitude (very useful) and maybe Elixir of Agility (quite useless). You can also buy Oracle's Elixir. If there is some champion with invisibility in enemy team, the advantage is obvious. Another usage of Oracle's Elixir is finding (and destroying) enemy Sight Wards and Vision Wards.
If you have full build and you buy elixir, Karthus automatically drinks it; but you have to be alive (so you can't buy any elixir with full build while you're dead). Another supreme ultimate effect of all elixirs is that you will radiantly shine like a christmas tree xD
The game still doesn't finish and you have some free gold? You can sell your Rod of Ages and/or Sorcerer's Shoes to buy another Rabadon's Deathcap, Void Staff or Rylai's Crystal Scepter instead. Your choice is the same as before when you were deciding among those items. Of course, you won't get neither second Void Staff nor second Rylai's Crystal Scepter if you already have one.
There are some items that I pick according to actual game situation. I recommend you to use them if you think that you can use them better than something from the basic build.
I don't like this item. If I overstate, I can say that if Karthus needs this item, it's discharged; if it's charged, Karthus doesn't really need it. I know that this is very simplified point of view but it's true very often. But the main reason why I don't prefer Mejai's Soulstealer for Karthus is that it doesn't complement his Death Defied and my playstyle (I will explain later in detail).
Edit (24.10. 2011): I have re-evaluate my oppinion for this item. It might be handy in some games. If you feel in early game that you are albe to get a lot of kills or assist, build Mejai's Soulstealer especially if your team is failing and you are supposed to carry the game. In that cases I build Mejai's Soulstealer before (or rarely after) Rod of Ages and I follow this build with just one exception - I don't build the last item. Why Mejai's Soulstealer before Rod of Ages? Well, it's just because in such early game (before you get Rod of Ages) you don't know if you are able to get so many kills or assists and if you are supposed to carry the game.
Edit (1.12. 2011): I buy this item in almost every game. The reason is that it's quite easy to stack it with Karthus. Even if you are dying a lot and even if you by accident don't kill a lot, you can always stack it by assists which you simply must have if your team has some kills.
Well.... in case of total despair which can be caused by late-game Veigar and/or extremely fed (burst) mage like LeBlanc (but I don't think she is able to be fed so much), Brand, Malzahar, Vladimir or Kassadin. That definitely doesn't mean you should buy Banshee's Veil in every game against fed mage (rather try to kill him than secure yourself; even if you resist him, you get killed by some AD champion).
Not taken items
I also consider to build following items but finally I decided to skip them. Feel free to use them if they suit to your playstyle but I recommend you to read my reasons why I don't build them.
I have tried to play those items in my build and I can't say that they all are bad in general; I just found some other items which fits better to my Karthus.
This is very good item for two reasons - it gives you AP and mana. Due to Defile's passive, potential blue buff (if your jungler gives it to you) or potential Clarity (if you decide to pick Clarity) Karthus almost never runs out of mana. And what about AP? Well, let's calculate it. You can definitely reach the highes AP in the game by using this item. 1x Rabadon's Deathcap + 5x Archangel's Staff + fully maxed runes and masteries = (155 (flat Rabadon's Deathcap) + 5 * 45 (flat Archangel's Staff) + 14 (masteries) + 34 (runes) + 5 * 0.03 * (1368 + 1000 + 5 * 400) (mana bonus Archangel's Staff)) * 1.3 (percentage Archangel's Staff) = 1408 AP - AWESOME. But there are two catches.
With such a beautiful build you aren't able to get some more beautiful items like Zhonya's Hourglass (as well as some less cool but still handy items - e.g. Will of the Ancients) and/or you lose your potentiality to breakdown enemy's magic resist by Void Staff but it isn't the main problem.
The most important disadvantage is that Archangel's Staff is super-stackable which means that every new Archangel's Staff enhance the old ones (new Archangel's Staff gives more mana to Karthus - so each his Archangel's Staff gives more AP) i.e. if you have just one or two Archangel's Staffs, AP bonus is quite low. Because we care about AP now, let's compare Rabadon's Deathcaps with Archangel's Staffs. Assume simplified game model in which you earn 300 gold per one game minute (in losing games you might earn less, in winning games you can earn more, maybe much more, but if you can earn e.g. 400-450 gold per minute, you win regardless your build). Rabadon's Deathcap is your first item (every build with Archangel's Staffs gives more AP if there is at least one Rabadon's Deathcap) reached in 12 minute (AP = 201). If you are building Rabadon's Deathcaps, you buy Blasting Wand in 15 minute (AP = 253), Needlessly Large Rod in 21 minute (AP = 357), Rabadon's Deathcap in 24 minute (AP = 403), Blasting Wand in 27 minute (AP = 455), Needlessly Large Rod in 33 minute (AP = 559), Rabadon's Deathcap in 36 minute (AP = 604), Blasting Wand in 39 minute (AP = 656), Needlessly Large Rod in 45 minute (AP = 760), Rabadon's Deathcap in 48 minute (AP = 806), Blasting Wand in 51 minute (AP = 858), Needlessly Large Rod in 57 minute (AP = 962), Rabadon's Deathcap in 60 minute (AP = 1007), Blasting Wand in 63 minute (AP = 1059), Needlessly Large Rod in 69 minute (AP = 1163) and Rabadon's Deathcap in (potential) 72 minute (AP = 1209). If you are building Archangel's Staffs, you buy Blasting Wand in 15 minute (AP = 253), Archangel's Staff in 22 minute (AP = 328), Blasting Wand in 25 minute (AP = 387), Archangel's Staff in 32 minute (AP = 534), Blasting Wand in 35 minute (AP = 600), Archangel's Staff in 41 minute (AP = 794), Blasting Wand in 44 minute (AP = 846), Archangel's Staff in 51 minute (AP = 1054), Blasting Wand in 54 minute (AP = 1106) and Archangel's Staff in 60 minute (AP = 1345). Well, as you can see, the build with Rabadon's Deathcap gives you higher AP till (approximately) 32 or 36 game-minute according to this model (you can see that it can't be said exactly because of both build are quite balanced between 32 and 36 game-minute). But all inaccuracies in that model advantage Archangel's Staff build: it's assumed that you charge Archangel's Staff each 4 second since 12 game-minute (which is quite optimistic estimation), Karthus' mana is as high as he was level 18 all the time, there is calculated no time for starting items like Sorcerer's Shoes (if you buy Sorcerer's Shoes before your first Rabadon's Deathcap you shift whole the simulation time forward), there is also premise that you earn constant amount of gold every minute but your earnings grows (moreover there isn't accounted that you earn more with higher AP), and there is assumed pure Rabadon's Deathcap build which isn't probably completely optimal (yes, it's optimal for AP but as long as you consider it as better than Archangel's Staff build, you have to also consider every superior build as better). If you account all that model's inaccuracies, you can maybe say that Archangel's Staff build is better since 40 game-minute. But majority of games is decided before that time, so I consider pure Rabadon's Deathcap build (and any other superior build) as better than full Archangel's Staff build. Moreover, Karthus isn't super-late-game hero (he is very strong whole the game but he excels over other champions the most before all builds are completed), so I think it's better to boost him as soon as possible to crush the enemies or at least overfarm them.
Another idea is to play just one Archangel's Staff. But one Archangel's Staff gives you only 166 AP (even fully charged and with bonus from Rabadon's Deathcap). Another Rabadon's Deathcap gives you 201 AP (well, it doesn't give you any mana but with your Defile's passive, potential blue buff or potential Clarity, you don't need it).
Quite good item but not for Karthus I think. 70 AP is not much (almost every AP item gives you more). Magic resist is unnecessary luxury that you can't afford unless Veigar is in enemy team (in late-game he is able to instakill you; to prevent it you have to build some magic resist). Magic resist reduction aura is great but your tank or support can carry it for you (just in case they can use Abyssal Scepter's AP bonus - of course).
I think this item is good (especially with your Lay Wastes) but I personaly don't like neither physical attacking nor Lich Bane. If you don't share my attitude, feel free to build Lich Bane for your Karthus; he is a lich anyway xD
I really appreciate graphic designers of League of Legends for this item. I just don't get how can somebody design something such cool. If you combine Hextech Revolver with Bilgewater Cutlass, what else you can get if not Hextech Gunblade - isn't that AWESOME? Since I'm fascinated by such creative idea, I wonder that many summoners (except e.g. Akali's players) don't even noticed Hextech Gunblade's appearance (or the reason why it looks just like that). When I showed it to my wife, she tells me "Wow! I finally know what's hidden inside the Will of the Ancients which is made of Amplifying Tome and Hextech Revolver!" She meant something like this xD
But why I consider to build Hextech Gunblades for Karthus? Well, it's the only item with not unique spell vamp. I have tried many builds for Karthus and I just don't believe anyone who tells me "it doesn't work" - I rather try it anyway. But.... this really doesn't work (as I expected - but it's free to try it) because of Hextech Gunblade's cost. Spellvamp is good but you also pay for useless AD and lifesteal.
60 AP is too low. You don't need mana regen due to your Defile's passive, potential blue buff or potential Clarity. CDR is good but Lay Waste's cooldown is less than one second and Defile has no cooldown; overmore you can get CDR from blue buff. The damage is also good but if you play such a killer as Karthus is, I recommend to use items to improve his spells rather than those with direct damage.
Something like Banshee's Veil against ADs. In general - don't buy it unless you are forced to do that. You don't really need mana. CDR is good but it not worth 2475 gold and item-slot. Armor and attack speed reduction is good but your mission is to kill enemies (not just debuff them). Frozen Heart gives you no AP at all so think twice before you buy it (it's much better to kill your enemy due to e.g. Void Staff than defend yourself by Frozen Heart).
I tried to build Guardian Angel because I wanted to piss my enemies off by "immortal" Karthus (he get killed, raised by Guardian Angel, killed again, ghosted by Death Defied and still fighting). Although it gives you some survivability, it doesn't worth one item-slot and 2600 gold. Maybe if you jungle with Karthus and you buy some armor anyway, you can upgrade.... maybe.... but I doubt about it's sense. In every way, I'm still trying to find some uncommon but working builds for Karthus and I'm going to experiment with items like Guardian Angel, Frozen Heart, Abyssal Scepter, Shurelya's Reverie and/or Warmog's Armor. If I find something functioning, I will write about it.
Masteries (edited 18.11. 2011 because of new mastery tree)
Since new season two mastery tree was released, 9-0-21 is not so clear choice because of quite good 21-0-9 possibility; despite that I still consider 9-0-21 as best mastery configuration for most mages but I am not so sure anymore (and I play some other mages with 21-0-9).
Offensive mastery branch is quite clear. Put four points into Mental Force ; alternatively you can put one of that points into Summoner's Wrath (if you use Exhaust or Ignite for some indefinite reason). There is useless Alacrity and Demolitionist or useful Sorcery in the second line; don't touch Alacrity and Demolitionist and put four more points into Sorcery . There is still one point left for the third line. Arcane Knowledge is definitely the best choice ( Deadliness and Weapon Expertise are completely useless; Havoc is acceptable but not as good as Arcane Knowledge which can be completed just by 1 point).
Utility mastery branch is more complicated. At first let's clarify what masteries are definitely the best for Karthus. I prefer Awareness , Mastermind , Intelligence and Transmutation (11 points total). Then you can improve your summoner spells.... Summoner's Insight enhances your Flash, Teleport and Clarity, whatever you play (so 12 points total). You have 9 free points now but you have to put at least three of them into the first line and at least another four points into the first or second line and minimally one point into first, second or third line. There are three possibilities in the first line: Expanded Mind , Good Hands or Improved Recall ; none of them is bad but I prefer three points to Expanded Mind and one point into Improved Recall (16 points total). Expanded Mind is great primarily due to Meditation which helps you to be very aggressive in early game. Improved Recall could help you very much in some rare cases (you are out of mana, your enemy out of health but still living, you both recall home and you want to cast Requiem then OR it can generally save you from being catch in the last second of teleporting). There is great Meditation and Swiftness in second line. I prefer Meditation because you can throw tons of Lay Wastes in early game (by early game aggressivity you can take the maximum advantage of Karthus), so put three points there (19 points total). One of the remaining two points you have to put into first three lines, the second point anywhere. I was choosing between Swiftness , Runic Affinity and Perseverence . Two points in Swiftness gives you only 1% movement speed so I decided to put one point into Runic Affinity and one point into Perseverence (now finally 21 points total).
I want to especially mention Awareness which I consider as great mastery. I think that four points in Awareness is a must. It helps you very much due to your hero and playstyle. You can easily overfarm and/or overlevel your enemies with Karthus. The bigger difference between you and enemy the better; Awareness gives you a lot. The main reason for not picking 21-0-9 is that you can't pick Awareness . On the other hand, there is Sage which I don't pick because of 40 experiences is really little (for comparison you need 280 xps for champion's level 2, 6480 xps for champion's level 10 or 19720 xps for level 18).
Glyphs: AP is the best offensive possibility that can glyphs give you. You can choose between Greater Glyph of Ability Power or Greater Glyph of Scaling Ability Power. Flat bonus -obviously- gives you more advantage for early-game; per-level bonus is better for late-game. You can hypothetically calculate that Greater Glyph of Ability Power is better until level 6 and Greater Glyph of Scaling Ability Power is better afterwards (0.99 / 0.17 = 5.82). Well, it's quite true but you should also include that AP +X at level 1 is stronger than AP +X (the same X) at e.g. level 6 because the more AP you have the less is the +X for you. Because of my playstyle (overlevel and overfarm your enemies as soon as possible) I choose flat bonus - i.e. 9x Greater Glyph of Ability Power but feel free to pick Greater Glyph of Scaling Ability Power or to combine both types.
Marks: Magic Penetration is the best choice for offense because AP marks are weaker than AP glyphs. The only MP rune is Greater Mark of Magic Penetration; pick it 9x.
Seals: There is no strong direct offensive bonus so you can choose between low AP bonus or (some) high defensive bonus. I prefer health because Karthus is dps mage; it means that he can deal as heavy damage as long he can stay in combat. Now you can choose flat or per-level bonus. Unlike with glyphs where I'm not sure what bonus is better, I'm quite sure here - pick flat even if it gives you less health since level 5. 9x Greater Seal of Health gives you 48.15 HPs which is 13% at level 1 or 2% at level 18 with this build. 9x Greater Seal of Vitality gives you 9.72 HPs at level 1 or 174.96 HPs at level 18 which is 2% at level 1 but just 7% at level 18 (with Rylai's Crystal Scepter even less than 7%). Overmore I still prefer early-game boost which leads to overleveling and/or overfarming enemies than direct late-game boost.
Quintessence: When I was choosing my quintessences, I considered Greater Quintessence of Magic Penetration, Greater Quintessence of Ability Power, Greater Quintessence of Scaling Ability Power, Greater Quintessence of Health, Greater Quintessence of Vitality, Greater Quintessence of Gold, Greater Quintessence of Cooldown Reduction, Greater Quintessence of Experience, Greater Quintessence of Movement Speed and Greater Quintessence of Scaling Cooldown Reduction. At first I discarded Greater Quintessence of Scaling Ability Power and Greater Quintessence of Vitality because I prefer their flat version (already explained). Then I eliminated Greater Quintessence of Magic Penetration (because I already have almost 30 MP from Sorcerer's Shoes and Greater Mark of Magic Penetration; that's enough for breaking basic MR (30) and I can't assume (in advance) that there will be many enemies with more MR in every game), Greater Quintessence of Gold (because Karthus is excelent farmer), Greater Quintessence of Movement Speed (because I think that this rune's bonus is quite low in comparation with other runes), Greater Quintessence of Cooldown Reduction and Greater Quintessence of Scaling Cooldown Reduction (because Karthus has no problem with cooldowns). There are three runes left: Greater Quintessence of Ability Power, Greater Quintessence of Health and Greater Quintessence of Experience. It's very hard to decide but I finally pick 1x Greater Quintessence of Ability Power and 2x Greater Quintessence of Health but I'm not sure about it; I consider all those runes suitable.
Standard offensive pattern
When you want to kill your enemy (e.g. your enemy is on low health after your lane-harassing or he's faraway from his nearest tower), follow this pattern; it's allways the same but some steps which you can skip in some cases (described in brackets):
- (if you have the Wall of Pain, mostly level 4+) Start your attack by slowing the enemy and lowering his magic resist. Where to put your Wall of Pain? Well, if you think your enemy is going to flee, block his escaping route (if there are more escape possibilities, wait until he chooses one if your waiting doesn't let him escape). Otherwise (e.g. he strikes back), try to put the Wall of Pain on him directly (if you miss him, move yourself to lure him into your wall; the most players pass it in the fray). Your target might be (temporarily) spell (or slow) immune (e.g. Morgana while shielded by the Black Shield); annihilate his immunity (or wait until it fades out) before casting the wall.
- (in any case) Predict your target's movement to spam him by Lay Wastes. Overmore, try to throw your Lay Wastes to hit your target only (for double damage). If you both are moving, it needs some practice to master your hitting skill (you should also know the map very well in every detail for better prediction of target's turning away).
- (if you have the Defile and enough mana for it's long-run) Turn on your Defile; then watch your mana and keep planning how much time you are able to hold your Defile running. I strongly recommend to deal with your Defile safely because nothing can piss Karthus' player off as much as the situation in which you can easily kill with some mana but you have to stay there like an idiot because of mana lack).
- (if your target flees out on low health and you can use your Requiem) Find some safe place for your channel. You have a lot of time to do that; enemy's recall lasts 8 seconds and he needs something about 1 more second (this time differs case by case) of staying in his base to heal enough to resist your Requiem. Your channel lasts just 3 seconds which gives you approximately 6 seconds to find some good place for channeling. Don't underestimate that; most champions can interrupt your channel by some cc (and every champion is albe to interrupt it by killing you xD). Although you should wait because of your popositioning, don't delay with no reason (the longer you wait the more time for your target's healing, shielding etc.).
As I said, I consider Death Defied as the best passive in the game. Even if you don't think so, you will probably agree that it's one of the best passives. Well, take advantage of your Death Defied by using it! Although this sounds crazy and there is very thin line between idiocy and maximalization advantages of Death Defile, I create my playstyle according to Karthus's passive.
If you're thinking that I'm telling you to die, it's might be partially true BUT there is another way how to use Death Defied to eliminate (or at least minimalize) risc of any actions; Death Defied makes some very dangerous and risky situations safe (e.g. fighting with melee comparable enemy while you aren't able to estimate who win because his health goes down as fast as yours - it's risky without Death Defield but with it you know that you can kill him even if he kills you first). Of course, there is even more offensive way how to use Death Defied (you can sacrifice yourself to kill something that worth it - but you have to be sure by that kill or -in most cases- kills).
You are overpowered as hell: Lay Waste, potential blue buff or potential Clarity (you can Lay Waste every target you see and you won't run out of mana) for offense, flat runes with MP, AP and HP (btw. +26% HPs at level 1) for.... yes, of course, more offense (as I mentioned - the more health the higher damage output total), Death Defied (for.... no, not for offense now, Death Defied is just for sure) and Health Potions which allows you to stay on lane even after you get hit during Lay Wasteing your enemy to send him home to his base.
If you're solo-laning (and you probably are), it's not about early kills but about overleveling your enemy and farming creeps. Use your awesome offensive potential to send him base and/or keep him out of last-hitting range. If you are able to kill him safe, do that (of course) but if you send him out of lane for few times, you overlevel and overfarm him which means you kill him anyway (sooner or later).
If you're laning with partner (but that isn't optimal role for you), you can get first blood quite easily because of you're excelent damage dealer. But don't forget your farm over the harassing. Karthus is item dependent and it doesn't change just because he is laning with partner xD. Because of your awesome farm, you may starve you laning partner. If he asks you to let him some creeps to kill, do it (if you think you can use the gold better, give him some minions anyway - it's better than quarreling team).
Death Defied in early-game: You can use it for 1v1 balanced fight as I mentioned earlier (if you win, you survive and get kill; otherwise you both die) but don't do that before first blood (for obvious reason). Proper using of Death Defied is definitely NOT fighting in style "enemy kills me but I probably kill him as well"! The right usage is "I kill him anyway and he will maybe kill me - or maybe not".
You are fed now. Even if you are losing the game, you should be fed at least a little. Since this build is specialized for early-game and mid-game (flat runes, no Archangel's Staff, Health Potions, maybe potential Clarity), you are farmed well and your score is probably positive now. You can probably face every enemy in 1 vs. 1 fight and win (mostly you can defeat even such champions like Vayne, Warwick, Tryndamere, Master Yi, Jax, Renekton, Udyr or Xin Zhao).
Besides 1v1 fighting Karthus is one of the best teamfighters - that start to show at the mid-game. Try to go in for every bigger skirmish but don't forget to farm creeps because you need it. I usually get the enemy mid tower down (if I'm not able to do it myself, I call for help because it's important for me to get rid of that tower) so I can play by this way: I farm mid-creeps quickly (more than one wave) to push up to their inner tower (of course, I don't go so deep without teammates but I send there some minions by clearing enemy minions out); it gives me some time for fighting in nearby teamfights (because I wander around mid, I'm in good position for moving into any location where I predict potential opening teamfight); then I often return into the base (because of shopping and/or regenerating). Afterwards I start again: farm mid-creeps, predict teamfight around, fight, return etc.
This phase of the game puts to the test your attention. When I'm tired or inattentive, it appear the most in mid-game. Why it's so hard? You need farm creeps as well as fight in every bigger combat if possible - both is very important but the second thing has higher priority. At first you must avoid getting stuck neither on farming nor on waiting in bushes for passerby enemy. And secondly you are supposed to watch the minimap and forecast the places where you and your allies can overnumber enemies OR where the enemies can overnumber your teammates if you don't help them. Sight Wards are very helpful or even necessary if you lose the mid tower and enemies occupy your jungle. If the map isn't warded, ask your support (or somebody else capable) to manage that (if you ask your team for something, don't write anything like "Someone, please, buy wards." (because many people ignore global asks) but ask specific person like "Sona, please, could you buy some wards?"). If there is no such person, do it yourself. Although it's not optimal, it's better than have no Sight Ward at all. Since the mid-game requires your concentration, don't spam the chat even if you're just farming creeps, don't distract yourself and pay attention - you are supposed to watch the minimap all the time!! Well, you are supposed to do it during all the game (and not just for Requiem) but in the mid-game it's critical.
Death Defied in mid-game: You can use it as well as in the early-game but there are teamfights now. You can be extremely aggressive because of your Death Defied AND because you are fed now. Remember that just Death Defied doesn't suffice - if you aren't fed, you die and don't kill. Otherwise you can do awesome things in teamfights because of your passive. Don't fear - if your AP is high, your enemies receive heavy damage before they finally kill you; then you can finish them by your Requiem. I say "don't fear" and I mean it.... but I definitely don't mean "recklessly feed your enemy like an idiot every time you see somebody on low health who can be chased by youself deep into enemy territory" by that.
You are still fed but your enemies also by now. You aren't the king of 1 vs. 1 combats anymore. You just can't face heroes like Vayne, Warwick, Tryndamere or Master Yi in 1v1 fight; they just overwhelm you like nothing. This is not caused by this build, it's just Karthus' nature (even if you have Rabadon's Deathcap + 4x Archangel's Staff + Archangel's Staff OR Void Staff build, you can't kill any of that champions with their full build - so don't try that and specialize in such game aspects you can master). Avoid 1 vs. 1 combats (well, if you meet lonely Soraka, kill her, of course).
Despite that you are great late-game character because you're an excellent teamfighter. Stay with your team, keep yourself behind your tank and when you meet some enemies, try to deal as heavy damage as you can (it's especially great with Galio or Amumu; Karthus is able to crush enemies so easy while Idol of Durand or Curse of the Sad Mummy is activated).
And golden late-game rule: don't go anywhere alone! Your main (and maybe the only) purpose is to deal heavy global damage to many enemies at the same time. So don't chase loners (if you are alone), don't do anything except of defending your base (if you are alone), don't even go for blue if you are not covered by your allies, be patient. Karthus is also great base-defender; if enemy creeps are eating your base and your team needs to push at the same time, you are ideal defender because (on top of you aren't effective turret destroyer) you can bravely annihilate hordes of minions in almost no time and you are able to face almost every champion under your own turret (put the Wall of Pain across enemy's way to force him to cross it if he wants to reach your tower, turn your Defile on and spam Lay Wastes.... you can also use Zhonya's Hourglass; if you fail anyway, use Flash to escape to your base and return back with full mana and health).
Death Defied in late-game: You can use your passive as good as in mid-game with one difference - enemies probably focus you in the moment they see you and their crowd control might stop you before you reach the place where you want to die (for full Death Defied utilization); you can insure that by Flash and get more time by Zhonya's Hourglass activativation immediately after flash-in (if you delay, you might have no other chance due to cc).
This chapter is neither about positioning nor attacks coordinating but about something what I consider as much more important. It's about team-spirit. You might wonder why I write it into Karthus guide. I wasn't write it if I wouldn't see epic fails caused by ignorance of this principles almost in every game. Many people underestimate this but I'm convinced that many games was won or lost just because of this (I believe it's about -at least- every fifth game).
Before every sentense you write into the team-chat, you should ask yourself: What's my main objective? Win the game or find out who caused the fail? What's better - to win the game or to prove that I'm the LoL guru and my teammates are just newbies? Am I really a specialist for playing every hero and am I absolutely sure that "strange" build of my teammate is really bad (or I just can't see it's advantages)?
If you are clear about all that questions, you are ready to communicate xD. I admit that I'm phlegmatic person so it's easy for me to advise others but think about that even if you get angry (well.... maybe especially in that case).
I don't know how to say it all understandably and clearly - so I put it into points. There are three types of advices: what to realize, what to do and what not to do. If you find out something more, tell me about it, please, I will update it.
What to realize:
- Golden rule: it's impossible to fully focus on the game while flaming the chat. Well, I know one lady called Miryo who is able to fight, write, eat and pet her cat at the same time; I remember epic conversation between her and our teammate (all this happened in 10 seconds): "Why do you stop fighting everytime you write something?" "Well, maybe.... because I'm writting??" Miryo has scored a double kill! "FFS, am I not? xD" But I really don't know how she's doing this and I definitely don't know anybody else who can even compare to her paralelism xD.
- That you have never seen some build doesn't mean it doesn't work. Even if it doesn't and your teammate is convinced of opposite, you aren't able prove him you're right during the game.
- You should play (and you are really playing) sometimes with a good team, sometimes with a bad one. If your ELO goes down all the time and you are more often losing than winning because you always play with a "noob team", face the truth - you are that noob (there is no other likely option).
- There is some really tragical player in your team. There is no doubt, he is really terrible (e.g. he plays Tryndamere with 0-25 score). Ask yourself: how good are you? Imagine that the game assign you into the games against FNATIC. Wouldn't you be happy for 0-25? Moreover, there are probably some players that can be overwhelmingly defeated by that Tryndamere of your team. By the way - you have never ever played against much better players? If you are not among 1% best (or worst) players, there isn't any big difference between "pro" and "noob".
- If you meet overpowered chars often, consider the relativity of the power.... you might be underpowered player to see everything OP xD
- The best score definitely doesn't mean you're the best player but if your score is significantly better than score of any other player, your team probably follows your advices; even if you tell them to stop their flame - use this advantage.
- The vast majority of people do the best for winning.... unless you vulgary blame them; then they might start to intentionally waste the game.
- It's not wise to recede an idiot from the long-term point of view. But the game is short xD
- Appreciate your teammates whenever they do something well. Say "well done" or "good job" if they kill an enemy or destroy the turret. In case of some bigger success, tell them more! You may think that it's unnecessary or useless but everybody feels better when you praise him; and the better people feel the better they play.
- Apologize for your mistakes. Even if you don't care it's polite, you probably prevent your teammates' blaming (and when they are writting.... you know xD). This thought is based on idea that it's harder to blame somebody who apologizes than somebody arrogant.
- Be nice to your teammates; in every case. Whatever you say, you can say that nicely. If you want something, ask for it (not just declare that). If somebody does a favor for you, thank him. Why? It costs you nothing and it can help you! How does it help? Few examples: You want to get blue buff. You can say "MY GOLEM" or "Could I get that blue, please?" - what do you think, in which case is your chance better? Afterwards you can say nothing or "Thank you." - what do you think, in which case is your chance for the next blue better? I don't say that it ensures the blue for you but it may improve your chance.
- Try annihilate any conflict in your team before it becomes huge. If some of your teammates start a flame, calm them down. Don't elevate over them by phrases like "How old are you? 13?" Even if they really behave like 13 yo, decently ask them to stop. You can say e.g. "Please, stop it. Let's play rather." If they don't stop anyway, ask them again or warn them that they may lose just because of flaming. Even if it doesn't work, try to not join any side of that conflict because it gets bigger by that.
- When you write ss, ping the map or -even better- tell your team where the missing enemy is (if you know that). If your teammates forget to look into the chat and they die, it's their mistake and you can write "ffs read the chat omg stupid n00bs" but what's better - lose with excuse for noob team or win with a good team? If you can see e.g. two teammates pushing the top tower against two enemies and your mid-opponent goes there, ping "fall back" there (and write "ss", of course). If the situation on the lane is not so clear and your mid-opponent goes there, ping few times along his way (I am also used to write e.g. "ss - went bot" or "ss - went top").
- If your teammate wants something from you, do it unless you consider that for really bad idea. If he calls you e.g. to defend some tower and you're not sure if it's really necessary (you are thinking about e.g. defending another tower), do what he wants. There are two reasons for that: he will be willing to do something for you next time AND if your ally is true and you don't listen to him, it can start some quarrels in your team. This advice is only for cases when you aren't decided what to do and your teammate asks your for something; if you are sure what are you doing and your ally asks you for something pointless, ignore him.
- Stay with your team in mid-game and late-game. They need you for teamfights - you are probably the most valuable member of your team for that. Don't waste your potential by chasing loners if your team is e.g. pushing or defending. This is not just tactical advice - if you miss some important teamfight, your teammates get probably (rightly) angry for you.
- Support enemy team's quarrels. I know, it's quite low but it works. If some member of your opponent's team plays obviously worse than the others, use it for yourself. Many people call him noob etc. but I think it's much better to show compassion with other enemies. If you write e.g. "OMG, Tryndamere, you're noob.", other enemies may stand up for him (especially in the case that he has some friend there). But if you write something like "I know that, it's frustrating to play with such Tryndamere who waste your game." and in contrast with that give respect to their best player, it stimulates them to quarreling.
- Definitely forget phrase "noob team!" Just don't say that. Never. There is no situation for which is better to call your team noobs than don't call them noobs. Really. No situation. That simple it is xD
- Don't blame your teammates. Let's assume that somebody in your team is doing something really bad, e.g. he repeatedly tower-dives and dies there without getting kills. If you tell him something like "omg, uninstall, you ****ing noob", there is a real chance that he does it intentionally next time. But if you write something like "They are probably too fed, what about play more defensive now?" - he maybe really stops that.
- Never give up, never surrender. OK, this is quite exaggerated but the point is that it's so stupid to write "surrender in 20" since enemy's first blood. I have seen many games with score e.g. 2-10 finally won by the early-game losing team and many games lost just for "surrender in 20" stupid talking. People relent in their effort while they can see "surrender in 20" in their chat. Think about it by this way: you spend your time by losing game anyway; you can either waste your time by negativity or use it by trying to win.
- Don't threaten your teammates by reporting. Idiots don't care about that and polite people might get angry because of that. Even if I report very rarely (I think I have reported two people ever), I don't tell you to don't report (it's your decision). I just tell you to not threaten others by that.
- Try to not steal even if it's quite hard with Karthus. There are some cases where you can't avoid stealing even if you want to (e.g. you are casting Requiem to kill some player but you kill two or three instead). If you steal, (deeply) apologize for that.
- Don't blame anybody for stealing. I personaly think that 90% of steals are not intentional even if it might looks otherwise. There's very thin line between helping and stealing and especially in teamfights people don't think about stealing but about winning. Overmore, if you scold someone for a steal, it won't guarantee you that he won't do it again. If he steals intentionally, he just laughs you; otherwise he might get angry (and/or do it again).
- Don't analyze your teammates' builds in the team-chat. Even if it might be polite discussion, if distracts your and your teammates' concentration. Overmore, there is always some danger of quarreling. If you're really interested in some build, you can discuss it after the game.
- Don't behave arrogant even if your are xD. People don't like that. If you present yourself as the master of the world, your teammates may be satisfied when you die or something and they might help that (at least subconsciously).
Possible kill(s) for possible death - worth it?
As each high-damage-output and squishy champion, Karthus faces this decision quite often (and the more because of his Death Defied). There is no universal template by which you can decide right every time but I can give you a guide of some aspects which should affect your decision (let's assume you know it's kill-for-kill and you can decide if you both die or not):
- Killing spree. The longer killing spree the more gold. If you are on a long killing spree and enemy is not, don't feed him; otherwise grab your gold.
- Health status. If your enemy is on low health (e.g. going base) and you are not, kill-for-kill means you both leave the fight for some time but he was returning anyway. This is very important in laning-phase (if he returns and you stay, more gold and exps for you).
- Hero's role. Compare your benefits to your team with your target's benefits for his team - you are great teamfighter, so don't trade kill-for-kill in late-game with e.g. Caitlyn. In general, this rule is much more important in late-game.
- Feed rate. If you are the one who devastate enemy team the most, feed yourself even if you feed some enemy also. But don't feed enemies who are dangerous to your team when fed (e.g. Vayne). This rule conflicts the "killing spree" rule very often (but not always). Well, I said that there isn't any universal template.
- Killing chance. You can never be sure that it's really kill-for-kill, you can't see whole game-state. If you see some real chance to die without kill, don't do that - hidden aspects you can't see are mostly turned against yourself and helping the enemy.
- Map positions. If there is real chance for your allies to kill your target in case of your fail, don't risc and let them act (you can support them by more damage but carefuly).
- The need of defense. If enemy creeps are eating your base, defend and don't chase enemies (unless there is some special reason to do that - e.g. you can safely kill more than one enemy AND your teammates in the base can handle the situation).
- Chances improving. If you don't know if you are able to safely kill your target, let's make him come to some better place (closer to your turret or allies). Many players act quite stupid when the chances are balanced: if you chase them, they run away; if you run away, they chase you. I don't know why but it definitely works (not with everyone but with many). You can use it: fight your target for a while and run away then. He might follow you regardless his chance to win.
There are many (I think 32) beasts on the map; some benefits all the team, some gives you a buff and all of them gives you some exps and gold. You can sort those beasts into four classes:
- Dragon and Baron benefits all the team which kills him. If your support or jungler doesn't ward them, you should (but ask them to do it first). Don't place it right into the enterance but farther into the river. If your team get some kills in short time period, go and slay one of those beasts - (in mid-game) you can slay Dragon alone (but it costs a lot of mana) or (in late-game) Baron with your teammates (costs a lot of mana if you turn your Defile on). Since Karthus damages beasts just by Lay Wastes and Defile, he doesn't need to wait for cooldown if ambushed - i.e. you are ready to fight all the time but your allies are not (which is essential problem unless you use your advantage by telling your teammates to not use their long-cooldown abilities if there is threat of ambush - you can do great damage alone especially with blue buff). Wall of Pain is quite questionable spell in this point of view - you can lower beast's magic resist which is great but then you need to wait for cooldown in potential teamfight so I suggest cast the Wall of Pain once (just when the slaying begin), use it's advantage and save the next cooldown for potential teamfight (if you don't expect any fight at all, use your Wall of Pain on the beast every time you can). If you are ambushed while attacking Dragon or Baron, stop fighting the beast and focus your enemies. There is almost no chance for you to last-hit that beast because of junglers with Smite.
Edit (30.11. 2011): Since today's patch, armors and magic resistance of Baron and Dragon can't be lowered. Wall of Pain is no longer effective against those monsters; so don't cast it.
- Your golem and enemy golem gives you the blue buff which you can use very well but you don't depend on it. Definitely try to get it (unless your jungler needs it) but if someone asks you for the buff and you find his request meaningful, give him that buff. And -of course- don't miss any opportunity to get enemy buff. If your jungler calls you to get blue, do it and thank him very much (if you appreciate him, team morale raises as well as the chance that he gives you the buff again).
- Your lizard and enemy lizard can be slain for the red buff which you don't need but your ADs teammates do. Don't take this buff from your lizard unless your enemies can steal it. Don't take the buff from enemy lizard unless there are no teammates around. Red buff isn't completely useless for you but ADs utilize it better.
- All other mobs give you exps and gold when you slay them. Don't do it in your jungle in early-game because of your jungler starvation. You can farm them in mid-game and late-game but I do it quite rarely because I prefer creeps farming over mobs killing so I don't have much time for jungling.
Common mid-lane enemies (sorted from the deadliest to the most futile)
I played against many champions. Some of them I have met many times on the mid-lane so I experienced games against poor players as well as against good ones.
This is some mix of common mid-lane enemies from ranked games as well as normal games. In ranked games you meet mostly AP carries, in normal games you can meet even AD carries or some other fighters.
If he is played by good player (and he mostly is because he is not popular among newbies), he is definitely the deadliest champion against Karthus. He can stay out of your range, so you can't harass him. From safe distance he can Riftwalk-in and immediately cast Null Sphere, both spells damages you (which is not so bad) AND silences you (which makes you completely helpless). If you have enough health, Kassadin just walk away (you can follow him silenced but it has no sense if your Defile is off - which is because Kassadin chooses the time of his own attack). He repeats it untill your health is low - then he stays and continue attacking instead of walking away. If you fight Kassadin mid, I recommend you to switch the lane (solotop is much easier even against two champions) or call your jungler (or some laner) to gank (but Kassadin is extreme badly gankable - you need more ganks during a time).
Her single-target burst damage is awesome. Probably the highest in the game. She is early-game hero. She can silence you. Played by a good player, it's hell for Karthus. Although this doesn't look good, I invent a solution: Use her biggest weakness against her. And what's her Achilles heel? She is probably the worst farmer in the game. Level 1 Karthus has better farming potential than LeBlanc ever gets. If you kill her creeps as fast as you can, you force her to use Distortion or even Sigil of Silence to kill minions; while on cooldown, you harass -or kill- her. The obvious disadvantage of this strategy is that you have to push which makes you vulnerable for ganking. You have to always know where are the enemies who can hardly cc you before you are able to react; moreover, if you follow this strategy, you should play with one finger on Flash (of course, your Flash has to be ready; otherwise don't risc it). If there is some enemy jungler with good cc, don't risc pushing ( LeBlanc definitely kills you with anybody else; she almost kills you alone). Very positive aspect of laning against LeBlanc is that you overfarm her without any problem and even if you screw it up and she gets fed, she is quite weak in teamfights as well as in late-game generally.
He is very dangerous because of this skill-sequence: Lightning Rush, Electrical Surge and Slicing Maelstrom. It stuns and badly harms (or kills) you. Even if I don't like using Flash for defensive reasons, I safe it for escape his ulti if he's my laning opponent. He harasses by his Thundering Shuriken but you can dodge it if you pay attention. If you fail to dodge, fall back for a while to avoid Mark of the Storm's stun. Kennen gives you a mark every time he hits you; if you get third mark, you are stunned. If you're marked once, there are three marbles circulating around you and one of them is highlighted; if there are two marks on you, two marbles are highlighted (if there are three marks, white spiral appears above your head xD). How to defeat him? Harass him faster than he harasses you. When he is on low health AND you are not marked by Mark of the Storm, attack and kill (nothing special, just put the Wall of Pain behind him, turn your Defile on and spam Lay Wastes).
Her Shunpo's blinks are deadly. Much more dangerous than Ezreal's Arcane Shifts because of it's range. She can also use her Bouncing Bladess to harm you from a long distance. Because of her mobility, it's very hard to hit her by your Lay Waste. If you're trying to Lay Waste her, she very often blink straight on you by Shunpo; she avoid your damage and simultaneously damages you. I was thinking about some tactic against her and I invented this: if you estimate when Katarina wants to blink by Shunpo to you. Throw Lay Waste under your feets. If your estimation fails, you wasted just one Lay Waste; otherwise turn your Defile, put the Wall of Pain on Katarina (when I say "on" Katarina, I don't mean "behind" her; she may decide to fight and not move backwards but you need to lower her magic resist) and start Lay Wasteing her. Even if you execute this plan well, you have no guarantee to kill her and survive (especially if she uses Death Lotus). In this case you can use your Death Defied and/or Requiem and/or Flash to kill her and/or survive.
(added 29.1. 2012)
Although Fizz is not good harasser, he can kill you in almost no time by his great bursts. It's very easy to underestimate him because he does almost nothing until he suddenly kills you. His great mobility allows him to reach you in almost no time as well as it can be used as damned good escape mechanism. Fizz is very hard to gank, very hard to kill. He can escape even your Requiem using his Playful / Trickster (if timed well; which is not trivial but every experienced Fizz can do that). Generally, while you play against Fizz, care about your health status - even if he might look harmless (because he hasn't damaged you hardly till that time), he is able to dash at you and kill you in almost no time.
(added 8.11. 2011)
Xerath is good harasser as well as he is great in finishing you by some burst for very long distance. To avoid his harassing you must change your moving patterns. Your standard mid-laning moving is based on walking as random directions as you are able to lasthit creeps and harass your enemy. While laning against Xerath, you need to realize that moving directly to him or directly away from him is the same as staying in one place because Arcanopulse damages whole the line; you can protect yourself from being hit by never walking by straight lines from you to Xerath in any direction (I usually walk in somewhat randomized helixes around him). If you are marked by Mage Chains, never run straight away because he roots himself into the ground by his Locus of Power and easily stuns you (to kill yourself by Arcane Barrage). In any case of Xerath grounding himself by using Mage Chains, quickly decide if you want to attack him or fall back (but never by the straight line) and do it; just don't wander around like an idiot unless you wanna die xD.
Don't stay in her poison and you own her. It's easy until level 6. Then you must keep an eye on her Petrifying Gaze. You can easily harass her by Lay Wastes and keep her on low health. But don't be gready!! She can easily get you while countering your strike. If you attack her, you face her - so Petrifying Gaze stuns you. Just harass and harass from long distance. If you send her home, it's good. If you kill her, it's even better. But don't try to aggressively finish her unless she's on very low health.
Edit (29.1. 2012): Before I wrote this text, I have met many Cassiopeias but they all have been played kinda weak players. Really good Cassiopeia is much better than Karthus in lanning phase. She infects you by poison and fires many Twin Fangs in a short duration. If you meet really good Cassiopeia, your only chance is not to push your lane, stay back and wait for ganks.
You have to be patient while laning against Morgana. When you harass her, she restores her health by Soul Siphon. When you decide to attack her by your standard way (i.e. Wall of Pain + Defile + many Lay Wastes), she propably snares you by Dark Binding and/or protect herself by Black Shield. Although I almost always prefer ofense over defense, it just doesn't work against Morgana because of her defensive capabilities. Just harass her (don't move forwards) and wait until she throws Dark Binding and miss you. Then attack. If she runs on low health, don't rely on your Requiem because of her Black Shield.
Edit (4.10. 2011): I met some good Morganas in last few days in ranked games. I think that they were played by better players than I am but anyway - I shift Morgana few ranks higher AND I want to complete this text: when she gets some AP, you are very easy to kill especially when you chase her or -in general- you are attacking. Dark Binding + Tormented Soil + Soul Shackles gives you stun and heavy damage. If there are some enemy jungler or ganker around, you are definitely dead. Otherwise, you might decide to continue the fight (if it starts by your chasing/attacking, I assume that she is low-hp). In that case she uses her Black Shield and if she hits you by Dark Binding again, you are done AND she probably survive due to Black Shield. So if you face Morgana played by some good player who can use her Dark Binding well, don't chase, don't be aggressive! Even if she runs into bushes on low health, she might wait there for you to come closer to snare you by Dark Binding and finish you even if she is on very low health.
If you don't want to be swain (in the sense of slain xD), always dodge the Nevermove (do you remember Edgar Allan Poe's The Raven?) which is quite easy to do but if you fail after all, Swain harms you badly. He is very viable due to Ravenous Flock and he is albe to stay on lane for very long time because of Carrion Renewal. So harass him (and don't get harm) for very steadily and be patient. When you move forward, he will probably Nevermoves you. I think you can overfarm him better than kill him (but both is difficult).
If the player handles Anivia's passive-aggressive playstyle, she becomes quite hard foe. Don't be as aggressive as usual because of her Rebirth - even if you take her health out, you aren't able to kill the egg if she reaches her tower before. Your better chance is counterattack while she's nearby your turret. If she is played by better player than you are, don't try to kill her at all - just dodge her Flash Frost to avoid stun an harass her by Lay Waste as long as she harasses you.
He can be very dangerous in hands of good summoner. Brand's main combo is Conflagration to blaze you, Sear to stun (and immobilize) you, Pillar of Flame (which you can't dodge if stunned) to heavy damage you and finally Pyroclasm to finish you. You can counter the combo by two ways: stay back if you are ablaze (to avoid stun) or dodge his Sear. Many Brands try to hit you by the Pillar of Flame even if you are not stunned. If you are ready for that, it's very easy to get away of it's range before it damages you. Karthus vs. Brand combat is quite balanced which means victory for better player (or the one who makes less mistakes. Since Brand is awesome farmer, beware of being overfarmed - just kill creeps as fast as he does (or faster) instead of harassing him.
You know exactly how she can kill you but it's hard to prevent that. Her combo Flash + stun + Summon: Tibbers + Ignite + some less cool spell is quite easy to play so you face it even if you play against quite bad player (well, you can meet someone who isn't capable even throw this combo - but don't rely on it). How to counter it? I have tryed few options (and I have hoped I find some offensive one but I haven't) and the best for me is: Harass as much as you can until level 5-6, then keep yourself out of her range (and/or under tower) and be prepared for her Flash - if she flashes and you aren't under your turret, use your Flash to prevent hers.
Edit (29.1. 2012): If you dont allow her to overlevel or overfarm you (or even better - if you overfarm and overlevel her) and if you start your Rod of Ages by Ruby Crystal and Catalyst the Protector, she is not so big thread for you. Watch out for ganks (with her stun are ganks very dangerous) and don't be aggressive while her stun is charged. You can handle her quite easy with good playstyle (by handle I don't mean defeat but overfarm and survive).
There are two spells that he uses on the lane a lot: Malefic Visions and Call of the Void. Malefic Visions is his main farming spell; as it jumps from minion by minion, it restores Malzahar's mana which causes him to try to push Malefic Visions by attacking affected minion (especially in case of the minion with cannon who can easily stop it) so you can predict that and Lay Waste him. Call of the Void is easy-to-dodge spell (remember that good old trick - he might put the spell behind you, so don't instinctively fall back) but if you fail, it silences you (and you are helpless without your spells). Harass him and don't let him to harass you (dodge the Call of the Void and don't let the Malefic Visions to jump at you from a minion). When you decide to finish him off, don't forget his ulti - he can give you Call of the Void + Malefic Visions + Null Zone + Nether Grasp even if he is on low health.
Fiddlesticks is extremely skill-dependent hero. The difference between bad, average, good and excelent is enormous (i.e. greater than at other champions). He can give you hell but you can also farm him like a minion (literally). You are vulnerable by Dark Wind due to it's silence (and long range - it can jump over many creeps to you). Dreaded Terrify + Drain combo doesn't need any comment. There is one important advice: don't rely on your great damages when he Drains you. If you are locked-on by Drain, get out of there; don't face him by Lay Wastes unless he is on low health. His only long-range spell ( Dark Wind) has pretty slow cooldown which enables you to harass Fiddlesticks quite well. He restores his health by Draining creeps but he can't move while doing that - Lay Waste him as hard as you can. Crowstorm + Terrify + Drain (+ Dark Wind) = you die. So don't allow him to surprise you with Crowstorm: he must cast it hidden (otherwise you can just walk away - and he is obviously unskilled) - so if you can't see him, assume that he is in bushes around your lane.
Hard laning foe. You can harass him well due to your spell's higher range but he can drain health to prevent your harassment.... damned vampire xD. He has good damage output, you have probably better but he is more viable on the other hand. If you keep yourself in a safe distance, you can Lay Waste him but he restores his health. If you get close to him, you can die as well as he can. Don't forget the damage-echo of Hemoplague - you can prevent it by Zhonya's Hourglass.
It depends how good her player is. She can give you some hell but Karthus is probably stronger than her. So just avare Twilight Shroud - if she casts it, fall back (you don't need to run far, just don't stay next to it or even inside). You can Lay Waste her even if she is invisible - because of Lay Waste's cooldown and mana cost, you can shoot blindly. If she is on low health (and you are not) and hidding inside Twilight Shroud, just go-in and cover it by your Defile.
He is very hard to kill (because of his shield) but his damage output is quite low. If his shield is discharged, you can kill him relatively easily (the state of his shield you can see below his health - there where you have mana status). Since Mordekaiser obtains his shield by killing minions, you can her*** him well before he reaches your creeps - so do it (my rule is one Lay Waste before every creep wave). With potential Clarity you can also try to break through his shield by spamming Lay Wastes (but it's quite mana-challenging because you have to crush whole the shield before you even harm him but he can run away anytime and it may cause your mana wasting with no effect).
She can harass you a lot due to ball's wide range. Although she overcomes you by the range, you can deal bigger damage; play offensive to defeat her. Bear in mind that she can interrupt or prevent (or at least minimalize effect of) your Requiem by Command: Shockwave or Command: Protect. In close combat (i.e. she stays within your Defile's range) you should defeat her.
Since Fnatic Shushei shows Gragas to whole world, he become played more often than before. When you meet him on your lane, try to dodge his Barrel Rolls and harass him despite his Drunken Rage which makes you harassing harder. When you try to finish him, remember that he can rapidly speed up due to his Body Slam. Bear in mind that Explosive Cask knocks you back and interrupts your Requiem.
Her spells are quite long-ranged, Light Binding gives you annoying snare and Finales Funkeln has very short cooldown. That means she fights from distance and harass you slowly. You can help yourself by Health Potions and in every way try to get close to her. She's very squishy, so Wall of Pain, Defile and Lay Wastes every time she approaches (if she doesn't, use Flash). There are two important things: her Prismatic Barrier can counter your Requiem AND where you are recalling on low health, she might hit you by Finales Funkeln (you can minimalize the chance for this by recalling from jungle nearby your lane).
(added 29.1. 2012)
Viktor is not so big problem for Karthus. Just never let him to stun you (I think that walking out from his Gravity Field before it stuns you is always possible) and care about his Chaos Storm (it does huge dps damage and the silence makes you harmless). Since you can predict (and dodge) his Death Ray in many cases, try to overharass him; it's much better than direct 1v1 close combat (his Power Transfer is very good spell for that, Chaos Storm as well and it increase his chances to stun you by Gravity Field while you are standing next to him).
His Time Bombs are annoying but not even comparable with your Lay Wastes damage. Theirs range is also limited. But he mostly stays back till level 6. Then he is very hard to kill due to Chronoshift; you can easily die (or at least not kill) while doing it. I think that Zilean is most dangerous when you are trying to kill him. If you meet him on lane, don't be reckless and take it easy since he reaches level 6.
Just dodge her Javelin Toss / Takedown spear and harass. When you get her to low health, use your Wall of Pain and attack her; if you are next to her, Javelin Toss / Takedown doesn't deal as heavy damage.
Edit (29.1. 2012): While she is played by good player, she can be very annoying because of her mobility (and repeated switching between woman and cat).... in that case you can help yourself by casting your Wall of Pain for defensive reasons (but it's quite mana-starving; ask your jungler for blue).
When Vayne gets some gold, she is deadly foe. She has awesome damage and movement. If you fight her, remember that she can easily tower-dive (so don't stay under your turret on low health - she kills you). Because her range is shorter than yours, try to don't get close - harass her from a distance. When she activate Final Hour, try to get out (and "out" doesn't mean outer edge of your tower's range; hide deeper). If you are close to her, she can use Tumble to get an ideal position and Condemn to knock you to some obstacle. So don't get close unless she is on very low health.
He is quite difficult opponent because of his range which is longer than yours (since level 6 - so you can harass him well until level 6). His Missile Barrage reaches almost 40% more than your Lay Wastes so you can't fight him from distance. Because of long range of your Defile, you mostly fight like that; but now you can win only by: move towards, Wall of Pain behind him, turn on you Defile and Lay Waste him repeatedly. Before I realize that, I have suffered in long-range combats against Corki.
He can be quite annoying because of Arcane Shift. You can dodge most his abilities if you pay attention. Don't reach too close to him (the longer distance the more time for you to dodge his shoots). If you are returning to your base on low health, he probably blindly throws Trueshot Barrage so don't recall from the middle of your lane - do it somewhere where Ezreal's player doesn't expect you.
He can harass you by Wild Cards very well due to it's awesome range. You can dodge the card sometimes but it's not easy. You can't defeat him by harassing, it's much better to use your Wall of Pain and get closer. Just watch out for golden card of Pick A Card (by that "watch out" I don't think dodge it -you can't- but bear in mind that Twisted Fate has that stun which he can use e.g. when you are turret-diving.
She is not popular hero (not picked by many players) so you mostly don't meet her. But she is great farmer and harasser with wide range. Even if you dodge her Boomerang Blade (if you don't few times, you have a big problem), she can still harm you by Ricochet from a long distance. It's not good idea to harass her from a distance because she probably harms you more. Use your Wall of Pain to slow her, move forward and kill. Keep in mind her Spell Shield (especially when casting Requiem).
While Strut bonus active, it's hard to hit her by Lay Waste but you can autoattack her before you start to harass (well, she probably harm you when you get too close). If she throws Bullet Time, get out of it's range (if you are on low health or or badly positioned, use Flash). There is nothing surprising or unsolvable what she can do to you; she just deals a lot of damage but you deal more.
He can barely kill you but you hardly defend your tower because he can push as hell. Lay Waste his H-28G Evolution Turrets to get him out of mana (it costs him a lot) and farm his creeps fast to defend your turret. When you try to finish him, he probably UPGRADE!!! his H-28G Evolution Turret to slow you (it also improves his directly-damaging skills; you probably get hurt while killing him).
He is quite tanky in late-game but squishy early-game. That makes him easy to kill but his Rune Prison grants him good escape mechanism. In general, you should own him; you overcome him by damage as well as by range. Your farm is also much better. Be careful when you are tower-diving, he might jail you by the Rune Prison. If he is level 6+, don't occur in any group of your creeps. When he casts Desperate Power + Spell Flux, you receive multiple damage.
Although he is great late-game killer which can give you hell, he is quite weak in early-game and laning. Harass him a lot because he can't do anything serious to you. Dark Matter is easy to dodge spell (unless you are stunned) and Baleful Strike has lower damage output AND Veigar needs it for killing creeps (to increase his AP) not for harass you. If Veigar casts Event Horizont, try to not get stun - if you are inside, don't go to the edge until it fades out. But the higher level the bigger advantage he has over you.... finally he can instant-kill you - but not in laning phase.
Dodge the Piltover Peacemaker (you can see how she is aiming - it gives you enough time) and don't step (by mistake) into the Yordle Snap Trap (even if the muffin inside looks good xD) and it's very easy to play against Caitlin. But remember hers Ace in the Hole (don't slouch around on low health). If your health is full and Caitlin isn't nearby, step into Yordle Snap Trap to deactivate them.
If Ashe's player isn't an excelent one, it's probably free kill for you. If he is excelent, you don't kill him but he doesn't kill you as well. Your opponent have to be much better than you to balance Karthus vs. Ashe combat (otherwise Karthus owns Ashe). She is helpless until level 6, none of her skills is not even comparable with your Lay Wastes; if she runs away use Wall of Pain to slow her. When she reaches level 6, be careful because of her Enchanted Crystal Arrow (it stuns you for very long time because it's one of the longest stuns in the game). If you are on full health, she is probably not able to kill you even if Enchanted Crystal Arrow hits you but it's nice chance for her jungler and/or another ganker. Above that, be careful while pushing enemy tower ( Enchanted Crystal Arrow can stop you under enemy turret).
He is super-squishy and anti-melee. You can farm him like a minion (yes, this refers to his size - among others xD). Every Lay Waste causes heavy damage to him. Karthus is better than Teemo in damage, range, viability, everything.... just his Noxious Traps are quite annoying but in laning phase you can just follow creeps and/or watch Teemo when he plants his mushrooms.
Karthus' deadliest enemies
This is a list of the worst champions you can meet with Karthus. Champions in this list are superior to Karthus in some way - i.e. you have to be much better player to stand them. If you are as good player as they are, you have minimal chance to defeat them. I don't write here every champion who is better in something than Karthus but only the champions who are really superior to him.
He is the deadliest laner as well as the worst enemy in any phase of the game. He is able to silence you which makes you unarmed. He is also able to Riftwalk away anywhere anytime or jump at you from nowhere. Your only chance is turn your Defile on before he silences you but even if you accomplish that, it's probably not enough. I'm still finding some strategy against him but the only one I know is to keep away from him and stay with some AD who can kill him much better than Karthus.
Although Veigar is quite easy laning-opponent, he is one of the deadliest champions in the late game for Karthus. By his Primordial Burst you can be killed in almost no time (and he will probably keep in for you because of your AP). There are two possible defences but any of them isn't ideal: you can change your build (e.g. buy the Force of Nature as the last item) AND/OR meet Veigar in teamfights only (use Zhonya's Hourglass and deal as much damage as you can before you die).
My ranked stats with Karthus (open the pictures to see them in full size):
Some good games with Karthus (open the pictures to see them in full size):
And here is my best ranked game ever; I have carried all my team, I have achieved more than half of our total kills and I have scored double kill, triple kill, quadra kill and even penta kill in one single game xD
Thank you for reading my build, I hope you like it.
I will be glad for every meaningful comment; point out my errors (even that gramaticals), I will correct it (if I will agree with you; otherwise we can discuss that).