Ive been playing Karthus for about a month or two now as one of my main champions. And now that I feel confident enough I'm going to write my first build on Mobafire! :D

As a champion Karthus is very powerful at all stages in the game. But with this great power comes incredible squishynes in return. In this guide I'll go over everything you need to know about Karthus from basic mechanics to his more intricate game play. Like team fighting, farming, and mid laning.

I'd like to give some credits to my resources as well:
Formatting from Karthus - Karaoke
General information and numbers from Karthus Wiki

On a side note I apologize for the lack of diagrams and video content. But I hope it won't heavily affect your voting decision!

Pro's:

-He is very powerful at all stages of the game
-An excellent farmer
-Can get off very good harass
-Very good damage output in team fights along with 1 vs 1
-His Passive Death Defied makes him useful even after he dies by channeling his ultimate and spamming all his spells over a duration of 7 seconds
-His ultimate can guarantee kills across the map no matter where the opponent is.

Con's:

-When outnumbered even by a small amount it's almost guaranteed you will die
-Extremely item dependent with Archangels Staff
-VERY squishy during all stages of the game
-Difficult to mid with a hyper aggressive champions (Graves, Leblanc, Fizz, Corki, etc...)
-Top focus priority in team fights
-Somewhat dependent on blue buff

My items are rather traditional for Karthus except for Rylai's Crystal Scepter. A lot of people who play Karthus tend to get 2 Archangels. Although this is quite good if your completely dominating late game. But overall I don't recommend this type of item build because it con's out weigh the benefits imo.

The reason why I choose Rylai's Crystal Scepter is mostly for the survivability, and utility. For instance in a team fight scenario you are being focused you want to refrain for dying to early in the fight so you can put out more damaage. So what you do is you activate your Defile. With Rylai's Crystal Scepter and Defile radius you can slow everyone chasing you along with some Lay Waste kiting. This also works well if your wall of pain is on CD. Which in most cases will happen in a team fight. Also the bonus health and 80 ability power are awesome.

Some question getting Archangel's so fast because your mana stacks on Tear of the Goddess will be rather low. But if your constantly last hitting with your Lay Waste then it should reach around 100-200 stacks. On top of the 400 mana you already get from Archangels Staff you will have slightly increased damage along with a larger mana pool making for great lane sustain!

Some alternative items you can get are:

Will of the Ancients
Abyssal Mask
Rod of Ages

Both these items are good on Karthus as alternatives Abyssal Mask for magic resist, ap, and magic resist are and overall great bonus. Will of the Ancients can greatly increase your survivability in team fights as well. Rod of Ages is also a good item for Karthus giving him extra health, mana, and ability power.

Runes:

My Rune pages are rather typical for an AP carry with:

Greater Mark of Insight


Basic Magic Penetration for any mage not much to go over with this rune

Greater Seal of Clarity


Mana regen is your best bet in seals for this build as the only other item you have for regen is Archangels Staff and the once in a while blue buff.

Greater Glyph of Force


A total of ~27 bonus ap at level 18. Overall great because it goes well with your Rabadon's Deathcap making for quite a substantial increase in ap.

Greater Quintessence of Potency


+15 ap for your early game Lay Waste last hitting.

Rune Alternatives:

Greater Mark of Potency


You could use this as an alternative for early game dominance but I still believe Greater Mark of Magic Penetration is better even if you are going for that strategy.

Greater Glyph of Celerity


The only real reason you would want cool downs with Karthus is for his Requiem. But the cooldown on his Requiem isn't high enough for me to invest in this over Greater Glyph of Scaling Ability Power imo.

Greater Glyph of Potency


Same deal with the marks, only it gives you more flat ap.

Greater Quintessence of Fortitude


If you feel that you can't get over how squishy Karthus is early game you could use these but you will be missing out on that great early game ap boost from Greater Quintessence of Ability Power

Greater Quintessence of Insight


One of the best alternatives I can recommend tbh. Really nothing bad about this compared to Greater Quintessence of Ability Power so go for it!

Masteries:

As far as Masteries go I prefer 21 0 9. You can go the opposite 9 0 21 for the extra utility. But now that mages get amazing bonuses from the new mastery trees I really want to abuse 21 0 9 with all the AP carry's.

Summoner Spells

I choose this summoner spell for almost any champion that I lane in mid with. It gives you great map control and has multiple uses like Teli ganking and so forth. Overall a great summoner spell for any champion



Flash along with a well placed wall can save your but if you get ganked. It's also a nice way to clean up a kill in the event that they run out of you Defile radius.

Alternative Summoner Spells

Some Karthus's I've seen get this summoner spell but you don't need it to be honest. Don't forget Defile's toggle off passive gives you a certain amount of mana return each time you kill a champion, turret, or minion with a basic auto attack or a Lay Waste. Overall you don't really need this spell but it is pretty good for those clutch moments when you want mana for your ultimate.



This can help your escape and/or offense capabilities by quite a bit as this ability synergizes well with your Defile chasing and so on but Flash fills that role the same way to an extent.



Similar to Ghost this can increase your combat effectiveness as well by reducing magic resist with the appropriate masteries and a huge slow. Allowing you to stomp on your enemy with your Defile and Lay Waste. But your Wall of Pain fills this role quite well and on a way shorter cooldown.



Has great potential on Karthus but overall his Requiem fills this role as well.

Death Defied


Upon dying, Karthus enters a spirit form, which allows him to continue casting spells for 7 seconds. During this time, his abilities cost no mana.

-This happens every time you die (has no cooldown).
-Your Defile is automatically turned on.
-You must channel your Requiem before the last 3 seconds or else you will not be able too.
-You cannot use summoner spells while in the Death Defied state.

Lay Waste


Creates a delayed blast at the cursor position. After a half a second delay, it will deal magic damage to each nearby enemy. The damage is doubled if it hits only a single unit.
Cost: 20 / 26 / 32 / 38 / 44 mana
Magic Damage (Multiple Target): 40 / 60 / 80 / 100 / 120 (+0.3 per ability power)
Magic Damage (Single Target): 80 / 120 / 160 / 200 / 240 (+0.6 per ability power)

-Your main nuke
-Your main farming spell
-Can be used to gain a small amount of vision to scout a brush
-If it hits one target it is considered as single target spell (this means it will benefit from the full slow of Rylai's Crystal Scepter and 100% of your current amount of spell vamp).

Wall of Pain


Karthus creates a wall between 2 obelisks at target position. Any enemy passing this wall have their armor, magic resistance, and movement speed reduced for 5 seconds. The wall lasts 5 seconds.
Cost: 100 mana
Cooldown: 18 seconds
Wall Length: 800 / 900 / 1000 / 1100 / 1200
Armor and Magic Resist Reduction: 15 / 20 / 25 / 30 / 35
Slow: 40 / 50 / 60 / 70 / 80 %

-Can be used offensively or defensively
-Great ganking tool
-Can initiate team fights
-Great harassment combo with Lay Waste and Defile
-Can be used to gain a huge vision radius where it is placed

Defile
Defile size 64

(Toggle)
Off: When Karthus kills a unit, he restores mana.
On: Drains mana to deal magic damage to nearby enemies each second.
Radius of AoE: 550
Mana Restored: 20 / 27 / 34 / 41 / 48 per kill
Cost: 30 / 42 / 54 / 66 / 78 mana per second
Magic Damage: 30 / 50 / 70 / 90 / 110 (+0.25 per ability power)

-Mini version of Fiddlesticks Crowstorm.
-Great late game farming tool.
-Passive greatly increases lane sustain along with an Archangels Staff.
-AoE is great in team fights.
-Synergizes well with a Rylai's Crystal Scepter and Will of the Ancients.
-Discourages melee champions from attacking you because they know theres a high risk of them dying with you.
-Can still hit stealth champions like Evelynn and Twitch.

Requiem


After channeling for 3 seconds, Karthus globally deals magic damage to all enemy champions.
Cost: 150 / 175 / 200 mana
Cooldown: 180 / 150 / 120 seconds
Magic Damage: 250 / 400 / 550 (+0.6 per ability power)

-The ability that makes Karthus awesome!
-Can get you a lot of kills/assists during the course of a game.
-Synergizes well with Rylai's Crystal Scepter by applying a global slow
-Can change the tide of a team fight
-Can be countered by any spell shield like Sivir's Spell Shield, Nocturne's Shroud of Darkness, and the item Banshee's Veil

Farming & Laning:

When you play Karthus you should always be in mid lane. Farming with his Lay Waste is rather simple but a little awkward getting used to. In short place your Lay Waste in a position that it only hits the one minion you want to last hit. If you can perfect this method of farming you will find yourself greatly ahead of your opponent.

NEVER farm with your Defile on. It leaves you vulnerable to harass by being on top of the enemy minions, drains your mana way to fast, and doesn't have enough damage to even guarantee you'll get all the last hits. The only time you should do this ever is if you have Blue Buff.

As for blue buff if you have a jungle champion who doesn't need Blue Buff much like Tryndamere, or Shyvana ask them to help you get blue buff every so often as this buff is pretty important to Karthus. But after level 6 you can get blue buff all by yourself.

If your mid opponent is being extremely aggressive chances are he is losing in the "Creeps Slain" department. In mid lane harass is something Karthus is very good at but in the event where the opponents harass beats yours just sit back and farm. Because in the long run you will be ahead.

For harassment on your opponent throw out a single placed Lay Waste for maximum damage. If you want to take it a step further and have blue buff you can Wall of Pain---> Defile---> and spam Lay Waste on top of them. If this is done correctly you can get your opponent very low or even kill your mid opponent for first blood before level 6! You can still do this hyper aggression without blue buff but be careful with your Defile as it will drain your whole mana pool without your buff.

Requiem


The second you get your ultimate notify your team! And tell them to call for it if the enemy is low enough. If you have some good teammates this is your ticket to getting free kills across the map!
NOTE: Make sure if your in mid lane and they call for a Requiem you are out of sight of your mid opponent because he could potentially stop your ultimate channeling or even kill you in the process.

By this time you should have at least built and Archangels Staff and a pair of Sorcerer Shoes if you have been farming well.

Ganking:

If your lane is pushed now would be a good time to go for a gank. It could be in any side lane you want but I prefer to go to the lane with the most squishiest champion in it as it makes for an easy kill. For a standard ability combo I like to open up with my Wall of Pain as this reduces there resistance and slows them as well. Making your Lay Waste easier to land and do more damage!

Mid Game Team Fights:

Although you are not the initiator you do have a Wall of Pain that can initiate a team fight with a slow/debuff. You can also poke your enemies pre-fight as well with you Lay Waste.

The majority of team fights will happen either in the dragon river area, around the buffs, and when pushing there outer/inner turrets. When team fighting it is important that you stay alive as long as you can before dying. Some Karthus players suicide and think that it's ok because of Death Defied. But the question is would you rather do the most damage you can over the whole duration of a team fight? Or the 7 seconds you get in an immobile state? I say the whole duration of the fight!

During the fight you should have your Defile on as well as spamming your Lay Waste. Always try to place your Wall of Pain to slow there escape or making an advance either way it will leave them somewhat cornered. Again I want to caution you about Defile because If you leave it on for to long you won't have enough mana for your Wall of Pain of Requiem (if needed). Also try get the AD carry out of the fight by chasing him off with your Lay Waste or even killing it if you can. This will allow the rest of your team to easily stomp all over the enemy team. But never underestimate the power of the enemy AD carry because it can also do the same to you if your not careful.

In the case that you die in a team fight don't forget your Death Defied state gives you and extra 7 seconds to do damage so do as much damage as you can dont hold back!! Also you should consider using your Requiem if you believe it will help your team win the fight but it's really up to your discretion.

Pushing:

When pushing as Kathus your aim is to get all the farm and leave the lane. Pushing the turret entirely could get you killed so you should leave that to and AD character who can fill that role like Tryndamere, Twitch, Shaco, basically any AD with an escape mechanism. But then there are times when a turret has barely any health and you can get it down in a couple hits, this is up to you.

By this stage in the game I would recommend at least having everything built up to a Rylai's Crystal Scepter. If not you should at least be finishing your Rabadon's Deathcap.

Late Game Team Fights:

At this stage in the game I would highly recommend staying in the back letting your tank initiate, because the enemy has probably came up with counter measure to your Wall of Pain, being anything from Mercury Treads, or just simply staying out of range.

The majority of team fights during this stage will probably happen at Baron Nashor, Dragon, Buffs, and inhibitor turrets. As mentioned before try to stay alive as long as you can by kiting with your Rylai's Crystal Scepter and throwing up your Wall of Pain (since they should be focusing you down). Also unlike early game and mid game, it's most beneficial to use your Requiem during the fight because if your doing well it will greatly turn the fight in your favor and it does a ton of damage. But you need to be careful of the timing because you are extremely vulnerable during channel, or you could waste it entirely if you do it to early. I like to do it either when there enemy tank initiates or when they start focusing a teammate down. So either way I know they have committed to the fight.

Just to wrap it up I wanted to put in some little notes for things I couldn't really catorgorizee this is also where anything you want to add will go so make sure to leave a comment with anything you have to say!

When using Requiem make sure you take into account any way of countering it before its use I see a lot of Karthus who believe there ultimate 100% guarantee's a kill and end up wasting it over something like a simple Spell Shield or Banshee's Veil

As far as Jungling Karthus I dont recommend it much just because he is very item dependent and jungling isnt reliable enough in his case to get much farm. Over all jungling is good I guess but Mid lane is wayy better!