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Disclaimer: Don't actually build that way huehue.

Survivable Karthus



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This is the more traditional AP carry route. You want to be a bit more survivable here since you won't be able to instantly Revive upon death. You may very will still die, which can be a good thing on Karthus thanks to his Death Defied, but you generally want to die of "natural causes", per se, like with any other champion, not just run in the middle of a fight and suicide. You generally want to provide utility to your team with your Wall of Pain, and DPS down the enemy carries with your constant Lay Wastes and Defile. Exhaust will assist you in shutting out the enemy carries during fights, as well as help you get some kills during laning phase. When you are about to die, however, try to jump on the enemy carries or in the middle of the enemy team, so you can get the most out of your passive when you do end up dying. Once you die, use Requiem and laugh when you get a pentaquadrabajillionkill.


Suicide Karthus



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The main idea behind suicide Karthus is to be a squishy glass cannon with tons of AP, run into the middle of a fight, turn on Defile, and lower their whole team down, if not pick up a kill or two. Especially late game, when you have literally TONS of damage, this helps your team immensely, since now they can run in with full HP and easily finish off whatever didn't die. You continue to deal damage after you die too, since you should position yourself in the middle of the enemy team when you suicide in, or on top of the enemy carries if they are staying back. Then you ult when you are in Death Defied if you can pick up a kill or two, wait till you die, then Revive and Teleport in to the nearest creep, tower, or ward. If you are fighting in the jungle, make sure that your team places a ward near where the fight will be happening, or a major objective such as baron or dragon if a fight is happening around there and the enemy team could potentially do it if the fight goes haywire. Make sure to keep the ward in an obscure location if an enemy has an oracles (ie behind a wall, behind a buff, at the back of Baron, somewhere that a champion with an oracle's won't easily spot, like in the middle of a brush, a lane, or the river, or right in front of baron/dragon) so they don't kill it and block your methods of re-entrance. Then, you come back in the fight (at superman speed with upgraded Revive, and with a bit of extra health thanks to Revive), turn Defile on, and kill everything that didn't die last time around.

Remember, Revive is on a 9 minute cooldown. Don't use it if you won't get an assured kill, especially on a carry, when you Teleport back in, if only one or two people are alive and have to recall anyway, or if your team can do the rest of the work without you. Remember, you can only use it a few times a game, so you have to make sure it is worth it when you do.

You generally want to leave blue buff to another teammate that can use it, because with this build you will have an Archangel's Staff, and you will be running into the enemy team and dying at the beginning of a fight. However, in laning phase, since you most likely won't be suiciding, it would still be a good idea to pick up blue if possible.

Try not to play suicide Karthus if the enemy team has a lot of hard CC (more than 2), since they can easily stun you and disengage when you try to get into their whole team. It can still work out, but it will be a bit more difficult.

Greater Marks of Insight
All of Karthus's abilities are dealt as magic damage, and as such you want magic penetration. This extra MPen is very helpful along with your masteries (if you follow my setup), your Sorcerer's Shoes, and the MPen from the Void Staff in this build, which will allow you to deal more damage with your spells.


Greater Seal of Vitality
Greater Seals of Vitality
With these, it is quite an easy choice. Besides these being half the price of the flat Greater Seal of Health, they give around 4 times as much health at level 18. Scaling Seals of Vitality also have a larger amount of total HP per rune turnover once you hit level 5, which, since you are going mid, you hit quickly.

You want health seals to make her a little less squishy. In total, 9 of these seals give around 180 extra health at 18, which helps a lot considering you don't have to spend any gold in-game to get the extra HP.

You could also go with the Greater Seal of Armor if you notice the enemy is physical damage heavy (if playing draft mode). Armor seals also help you take reduced damage if you draw minion aggro, and most AP mids harass often with auto attacks -- having armor seals mitigates a bit of that damage. I still prefer HP/level seals more often than not, because it is effective against all types of damage, but these are a good alternative.

I get these runes over the Greater Seal of Scaling Mana Regeneration because of Defile and the item build. As long as you aren't flying Q's around randomly, even if you use some to harass the enemy champion (as you should), you will gain most of your mana used (with last hitting) back. You also either get a Tear of the Goddess into an Archangel's Staff, or a Catalyst the Protector into a Rod of Ages, both of which give a lot of mana and some mana regeneration (the "regeneration" from Catalyst/RoA is in the form of passive restore upon leveling). You will also most likely be getting blue buffs from your jungler.

In fights, whether you are playing suicide Karthus or not, there is a very good chance of you dying at some point if you are fulfilling your job of doing tons of damage to the enemy team (this requires you to either straight up run in and Defile, with suicide Karthus, or stay in close range of the enemy with constant Lay Wastes, and most likely run in and Defile at some point. If you are dead after a few seconds or even after an extended fight, or even if you survive, you won't need extra mana regen past your masteries and items (usually the passive of Defile isn't utilized in fights), and blue buff if you have it. Mana regen runes are usually most useful for lane sustain, which you don't need thanks to Defile.

They are still not bad runes to take, so if you have them and still feel like taking them for the increased mana regen, especially if you are newer and have trouble managing your mana, feel free to. But they really aren't necessary.



Greater Seals of Force
Since you don't need the extra survivability on suicide Karthus, I take some extra damage on him with these seals. If you feel you need the extra health regardless, feel free to take the Greater Seal of Vitality.



Greater Glyphs of Warding
These runes help a ton. You get 12 MR in total from the start, and since you will most likely be mid, this means you are negating quite a bit of damage from the enemy AP's spells, which helps a lot early on to let you lane longer. I get flat over scaling here since the scaling Greater Glyph of Scaling Magic Resist becomes more effective at level 9; however, by then, you will have some more health and tankiness anyway, from your items as well as naturally leveling up, so having a bit more MR won't be nearly as effective as having a large amount early game. You need to make sure you stay in lane and do not get pushed out early, since that can set you far behind and put your enemy way ahead. The larger amount of MR from these flat runes over the scaling ones early game help you achieve that, especially since Karthus is naturally pretty squishy and really needs to stay in lane and farm up.

Take these runes if you are against a high-burst match up and feel you will easily get pushed out of lane. For example, if you see a spam-heavy, bursty champion that you will be going against in draft mode, such as Ryze or LeBlanc or Veigar (see more counters in the "Laning" section), it might be worth it lose some damage but gain some MR so that you don't get pushed out of lane as easily. Likewise, if you are against an easy champion to bully early game, such as Ziggs, you may find the extra damage to prove more useful than a bit of reduce damage from his harass. It is mostly up to preference, playstyle, and the game situation. I tend to take MR blues on survivable Karthus to help you live a bit longer, and take less harass in lane to be able to stay and farm more. Like I said though, the runes are really up to preference and game situation (matchups, etc.).



Greater Glyphs of Force
These runes, which I take on my suicide Karthus build, work really well on him. They add 27 ability power at level 18, which amounts to a little over 35 with Rabadon's Deathcap, and taking masteries into account. This extra AP adds quite a bit of extra damage, especially consdering Karthus has such good scalings: the 35 AP adds a bit over 20 damage on EACH of his single target Q's, around 20 damage to his ultimate, and almost 10 damage extra per second on his Defile. This extra damage adds up to be quite a bit over time, and could make the difference between netting a kill or not.

I get these runes over the flat Greater Glyph of Ability Power because of the turnoff at level 7. At level 7, these runes deal the same exact damage as the flat Potency glyphs, and deal more at 8 and onwards. Since you will be in mid lane as Karthus, you will hit level 7 quite rapidly, and at 18, you will end up having 16 more ability power than you would from the flat Greater Glyph of Ability Power.

Get these runes if you don't feel you will get bullied out of lane easily and you think you can win your matchup pretty hard, or if you want to risk not doing as well early game in laning phase to have a bit of a stronger late game.



Greater Quintessences of Potency
These quints are amazing to take with Karthus, as well as every mid AP champion. These runes work with your Deathcap as well, just like the Greater Glyph of Scaling Ability Power. I take these over the scaling Greater Quintessence of Scaling Ability Power because they affect early game a lot (around 15 AP right off the bat, without any items, just from these runes!). Along with your Rabadon's Deathcap, these give a lot more AP to you than if you didn't take them with you, throughout the game. I take these instead of scaling for this extreme change early game. It would be one thing if scaling gave 3 times the amount of AP at level 18, and the turnover were really early game (like the Greater Glyph of Scaling Ability Power vs Greater Glyph of Ability Power); however, flats are much better in this case for quintessences. Once again, they give an immense amount of gold-free AP early game; at level 18, scaling only wind up giving around 8 more AP, which won't have nearly as much of an effect late game as 15 AP from the start does early game. As well, the more-AP-per-rune turnover for these runes is pretty late, at level 12. Even though this is not much of a problem if you are going mid, remember, the early game effect from these is immense.

Other viable alternatives could be the Greater Quintessence of Movement Speed, since the extra movement speed helps a lot early game for mobility and juking enemy spells, and the Greater Quintessence of Spell Vamp, which would give you a free 6% spell vamp off the bat, which would stay with you throughout the game, increasing your vamp from 20% with Will of the Ancients to 26%.

RECOMMENDED



Exhaust
This is a really nice spell to take on non-suicide Karthus. It lets you focus down an enemy carry in teamfights, and helps you net kills in lane by allowing you to easily hit Lay Wastes when your Wall of Pain is on cooldown, most likely granting you a kill or getting them low enough to ult. I take this over Ignite, because, as will be mentioned later in this section, Ignite is generally used on champions with combos and to help finish off enemies. However, Karthus deals sustained damage, and has his ultimate to finish champs off, so he really doesn't have much use for Ignite. As well, in fights, being suicide or non-suicide Karthus, you want to focus on bringing the enemy carries down, and Exhaust will help you a lot more with that task than Ignite (to land Q's when Wall of Pain is on cooldown, to reduce the damage they deal to you so you can kill them before they get you, etc.).



Flash
A very common spell on most champions. This spell lets you escape enemies, close any gaps, initiate any fights, get out of turret range, and more. Pretty standard, and should be taken if not running suicide Karthus (I don't take this on suicide Karthus because you need the combination of Revive + Teleport in order for the strategy to work).



Revive
Necessary spell if you are going to run suicide Karthus. As mentioned earlier, allows you to run in to a fight, deal tons of damage, die, and come back to finish the job up. Also grants some bonus health for a bit upon revival, which is a nice perk.



Teleport
Necessary to pull off your suicide Revive + Teleport combo: Revive after death, and teleport back into the fight. Also helpful for getting to dragon/baron fights if you are not there (say, at base), which can turn the tide of a fight for your team. Remember, you can use this spell without using Revive, however the reverse usually isn't true unless you forget to ult for some reason when you die and Revive to ult and get a pentakill. When not in teamfight stage, helpful for ganking other lanes (teleport to a ward in a brush or a creep or tower), defending other towers, and getting back to your lane immediately after you recall so you don't lose any farm.

MEH




NO...JUST...NO





Any spell not mentioned is a definite no-no for obvious reasons, too obvious to even spend time discussing.

Death Defied
This passive is great, and makes Karthus who he is. This passive is also what makes suicide Karthus viable. When you die, you still deal damage. Tons of it. It also lets you safely cast your ult, since nothing that can usually stop a channel can stop it when you are already technically dead. A terrific passive overall.



Lay Waste
This is your main harassing, farming, and damaging tool, all on a 1 second cooldown. Deals double damage to single targets, which makes it a superb ability to help you instantly kill enemy carries in lane and in fights. Also makes taking dragon and baron early game a breeze with your team.

To last hit with it, simply wait till creeps are almost going to die, and cast it under the creeps that are about to die. Keep in mind it has a 0.5 second delay, so if you notice creep attacks heading to the enemy creep, make sure to wait until they arrive to use Lay Waste or else they may end up taking the CS from you.

I learn this ability first so that from level 1, I can be harassing and farming safely. I max it first, since as mentioned, this is your core, your bread and butter, the ability that you will spam most often, the ability you use for harassing and farming; and as such, you want to get the most out of it as soon as possible.



Wall of Pain
Great utility ability. At max level, applies an 80% slow that diminishes over five seconds; also reduces enemy MR and armor. At level 1 however, the slow is still nice, at 40%. The wall also provides sight, so you can pop it in a bush, in baron pit, dragon pit, etc., to help your team steal, get sight on a possible gank incoming, etc. Can't be destroyed like a ward, so it will fully give sight for all five seconds that it's up.

This ability is mainly for utility; you want your damaging and sustain (more mana restory per level of Defile) abilities maxed out first. As such, I get one point in this ability at level 4 but wait to max it out last.



Defile
Karthus's signature AoE that deals a lot of damage per second. Combined with Lay Waste, this ability makes Karthus a DPSing AoE menace in fights before he ults, and stays in death if turned on before Death Defied activates. It also grants more mana per unit kill per level. Quickly turn on and off this ability to pick up some last hits that you would otherwise miss. Also good for AoE clearing creeps to push a lane out (to get a tower, roam and go gank, etc.).

If you do go OOM, you can last hit minions with your auto attacks to gain mana back.

For the most damage possible and to get the most lane sustain possible, especially in combination with your Catalyst of Aeons/ Tear of the Goddess and Hextech Revolver, I get my first point in this ability at level 2 and max it second, by level 13.



Requiem
One hell of an ultimate. This ability is the signature ult that hits everyone on the map. Someone thinks they get out with low health, only to notice the red line streaming above their head a second later, telling the inevitable.

This ult lets you finish off kills, assist other lanes without leaving your own, deal massive damage to the enemy in fights, even if it doesn't kill them, and makes you look really cool when you use it to killsteal a teammate who was clearly going to get the kill anyway (try not to do this, as the ability is on a really high cooldown, especially early game -- you need to make sure you don't waste it, since there's a chance you could be more useful with it later, only to find it is on cooldown).

Remember that this ultimate is CHANNELED, meaning that hard CC or death will interrupt it. Please do not cast it right before you die, or it will just cancel. Don't cast it in the middle of a fight when you are still alive, you will most likely get hard CCed and it will be canceled. If you are going to use it out of a fight, back up to a safe area such as a tower, to make sure the enemy doesn't notice you using it and cancel it. If you are using it in the middle of a fight, wait until either you die, since then it can't be canceled (make sure to cast it soon after Death Defied activates, however, since if you wait too long you won't be able to use it despite still being in Death Defied, as it is a 3 second channel and Death Defied only lasts for 7 seconds), or the enemies run from you/you run from the enemies.

In fights, you generally want to save this ability until you are safe in Death Defied mode, and finish off some kills/lower the enemies even more for your teammates to clean up. However, if you are in a fight and you were previously not able to kill an enemy carry without dying, or if you teammates keep dying without a certain player (usually a carry) being dead (ie they can't handle him), it may be a better idea to use your ultimate at the beginning of the fight to get the carry low as well as the rest of the enemy team. Remember -- your ultimate still deals damage if it doesn't kill someone! Although it always brings great satisfaction when you click R and get a triple kill, you don't need that to happen -- if you ult at the beginning of a fight or in the middle even, you still deal damage and get the whole enemy team relatively low, and as a result easier for your team to fight. It really depends on the game situation -- if your team is consistently losing fights and you are not able to finish the enemy carry off, or have to die to finish the enemy carry off, it might be a better idea to ult at the start of a fight. If your team is doing fine and isn't having too much difficulty in fights, feel free to run in, do your thing, and save your ult for once you go in Death Defied form. It's all situational and up to your best judgement, really.

Learn this ability whenever you can.

Skill Combo

You don't really have a "combo", per se. Just DPS down the enemy, with constant Lay Waste harass. Try to land it when they are away from creeps if possible, so it deals double damage to the enemy champ if it only hits them. Use Wall of Pain when you are ready to go in for the kill, and Exhaust if necessary (and you are playing survivable Karthus) once the slow ends. If they get away with enough HP for your Requiem to kill them, use it quickly. Make sure you are away from any source of death or hard CC when you do. Make sure you know the damage your ult does at all times, and notify your teammates of it too. Be mindful if someone gets away that you can assuredly kill them, because if you don't, you just wasted a ~3 minute cooldown ultimate.

If you are in a fight with multiple enemies, turn on Defile and Wall of Pain them, then constantly DPS the enemy carry with Lay Waste, and Exhaust them if need be and you have exhaust. Then, when they are low enough, and you can net one or more kills with your ult, stay clear from sources of death or hard CC if you aren't dead already, and pop your Requiem.

Remember that a silence is considered hard CC, not just a stun/knockup/fear. Try not to ult right next to a Soraka, Lulu, Malzahar, LeBlanc, Kassadin, Talon, Cho'Gath, Garen, Blitzcrank, etc.

Lay Waste

Counters

Champions that you generally want to avoid if possible (draft mode)

-Champions with single target abilities, ie. Ryze and LeBlanc -- they can easily melt down your health bar.
-Champions with hard CC, such as, once again, Ryze, LeBlanc, Veigar, Xerath, anyone with a silence (listed above), etc. -- can cancel your ult and set you up for some nasty harass and possibly open you up to a gank, of which Karthus is quite weak against, since he has no hard CC to deter the enemy ganker, only a slow with Wall of Pain and Exhaust if you are taking it; even worse if you are playing suicide Karthus and not taking Flash nor Exhaust (against champions with a lot of hard CC, that can easily disengage from you trying to get into the middle of their team, generally suicide Karthus is not a great idea, although it can still work).
-Champions that have huge burst/nukes, such as Xerath, LeBlanc, Kassadin, and Veigar -- they can shut you down completely before you have the chance to even do anything, because Karthus is so squishy (especially if playing suicide glass cannon Karthus)

Here is where I will tell you when to get what items, explain why I use the items that I use, and the effects of them on Karthus.

NOTE: Try to buy an Elixir of Brilliance whenever you recall right before you know a teamfight is going to happen and you have the extra gold, or if you just built or bought an item and have some gold to spend. You can even buy one of these elixirs if you recall and don't have enough gold for any other items that you want to get yet (just don't go overboard on the elixirs, as you still need enough gold to buy your main items). The elixir gives you some more AP and some CDR, which, for a measly 250 gold, doesn't hurt at all. This is especially helpful early game, where that 25 or so AP plus the CDR has a huge effect, when your AP is low and CDR is measly.

ANOTHER NOTE: If you still have some leftover gold after your purchases, and some open slots, ALWAYS TRY TO BUY WARDS! If you don't have leftover gold, try to make some the next time you go back! Stay in lane to get three more CS if you need some more gold for a ward! Lose a few health pots! Don't stay in the dark! Don't rely on your jungler to ward your lane if you are top or mid, although oftentimes they will. Buy some Sight Wards, you will not regret it! WARDS SAVE LIVES!

Early Game

Boots of Speed and 3 health potions
This is your core start with Karthus, and most champions in the current meta (minus supports). Since you have mana regen from your masteries and mana return from your Defile passive, that paired with the 3 health pots is enough sustain to keep you in lane for a while. The boots will later build into your Sorcerer's Shoes, and provide nice early movement speed to let you chase/escape enemies quicker, evade spells, and more.


Catalyst the Protector
Catalyst the Protector
First item to get or start building upon recall if you are playing survivable Karthus. Provides some nice health and mana, letting you get bullied a bit more, and some nice sustain from the passive upon leveling up. Will be build into a Rod of Ages later.



Tear of the Goddess
First item to get or start building upon if you are playing suicide Karthus. Gives you good mana sustain, which, when paired with your upcoming Hextech Revolver, lets you stay in lane almost infinitely, and allows you to start building up your tear mana which will stay when you convert it into an Archangel's Staff later on, giving you a nice boost of damage.



Hextech Revolver
Gives great sustain, which, combined with your masteries, Defile, and Catalyst the Protector or Tear of the Goddess, along with blue buff, allow you to outsustain your opponent and stay in lane to get kills and farm much longer. Will be built into a Will of the Ancients later.

If you are playing survivable Karthus, get this item AFTER your Rod of Ages -- you need it early on so that you get its full effects.



Sorcerer's Shoes
Gives you flat Magic Penetration, which, tied with your runes and masteries, will let you deal near-true damage to anyone before they get MRes, which usually isn't until mid-late game. Then, you will get a Void Staff to keep up with dealing as close to true damage as you possibly can. Once you get said Void Staff, you will be selling these shoes for Mercury's Treads if the enemy team is mostly tanky and building MR. The reason for doing this is because flat MPen is applied BEFORE percentage MPen, such as the Void Staff. To quote the League of Legends wikia, "Because the percentage magic penetration is applied last, its effectiveness is reduced the more flat magic penetration you already have or the more magic resistance reduction has been applied to the target. For example, if the target has 100 magic resist and you only have 20% magic penetration, you will penetrate 20 of the target's magic resistance. If you also have 20 magic resistance reduction, the same 20% magic penetration will only penetrate 16 of the target's magic resistance." So, although keeping Sorcerer's Shoes will allow you to still deal some more damage to your targets, the reduction of the effect of the Void Staff makes the Sorcerer's Shoes less useful than they were before the Void Staff, and ultimately, Mercury's Treads become a bit more useful at that point in the game if they are primarily tanky and building MR. However, since you generally die at some point in a fight as Karthus, the extra MR and tenacity isn't always necessary. Thus, only swap these out for Merc's later on if you are really getting bursted down by AP damage, or the enemy team is building a lot of MR (200+ on 2 or more targets, since then Void Staff will have full effect if you remove Sorcerer's Shoes).
If an enemy gets Mercury's Treads, these shoes along with your runes and masteries will counter the MRes from them and still reduce some base MRes.

I get these boots before my Rod of Ages because, to quote DuffTime,


By 2, he means of course Enhanced Movement 2, not literally 2 more movespeed. The extra MPen from Sorc's increases your damage by quite a bit early game, so you definitely want to get them before you finish your Rod.



Rod of Ages
For survivable Karthus. Builds from the Catalyst the Protector purchased earlier. It grants a nice amount of AP (80, 104 with Rabadon's Deathcap), as well as a large mana pool, which helps a lot to keep you in lane longer and constantly casting your Lay Waste for harass; it also gives you a hefty amount of HP, which lets you absorb some more damage before dying, possibly buying you enough time to net a kill or get in a good position for Death Defied. I get this item before Rabadon's Deathcap so that we have a higher chance of getting all that we can out of this item, since it takes 10 minutes after purchase to grant the full stats that it offers.



Hextech Revolver
Pick this up now if you are playing survivable Karthus.

Mid Game

Rabadon's Deathcap
This item is just amazing. You want to rush this item after the Rod of Ages (if playing survivable Karthus, or else just straight up rush it) for the insane amount of AP that it grants (140) plus the passive 30% increased total AP. Getting this item early also means that every item that grants AP that you get after it will have the additional 30% extra AP added to it right off the bat, along with any items you purchased beforehand. Adds around around 290 AP with the base AP + passive at the end of the survivable Karthus build, or over 300 with the suicide Karthus build, not including blue elixir or baron buff.

A must-have for Karthus, as well as for every other AP carry in the game.



Will of the Ancients
This item, in my opinion, is core on basically EVERY AP carry, ESPECIALLY Karthus. The stats it gives for the cheap price are just too hard to pass up, and they help the whole team, too!

Of course this item gives nice AP, and a good aura for your teammates, but the real thing that we are looking for here is the spell vamp. Obviously don't need a Hextech Gunblade, the only other item besides Will of the Ancients that provides spell vamp, since the AD and life steal, almost half of what you're paying for, aren't useful for us on Karthus, and the active isn't necessary when you already have a large slow and deal primarily AoE damage (although you deal a lot of single target damage with Lay Waste as well).

You want spell vamp on Karthus, because, combined with your mana sustain, you will have basically infinite sustain in lane and a lot of sustain in fights that will keep you alive longer. You may be thinking that spell vamp on Karthus is pretty underwhelming, since all of his damaging abilities are AoE, and only 33% effect is applied to AoE spells from Will of the Ancients. However, keep in mind, the 33% is applied to EACH TARGET SEPARATELY. Meaning, if you have three targets dealt the same amount of damage from an AoE ability versus one target dealt that damage from a single target ability, both abilities would heal for the same amount.

Now consider this. MORE than three targets. How about a whole team of five champions? How about a whole wave of 20 creeps?

You heal for a lot.

Since you have low cooldowns with your Lay Waste and constantly ticking Defile, you will consistantly be healing with this item.

If you have blue buff, you basically never have to recall once you have this item. If you are getting low, find a creep wave, and bingo, you are back at full health, and you have full mana from blue buff.

You can also kite enemy champions if you are getting chased down with low health -- slow them with Wall of Pain and constantly hit them with Lay Waste and Defile if they are in range. Eventually, if they chase you long enough, thanks to spell vamp, you will be higher health than them, and will be able to kill them.

A terrific purchase for Karthus.



Archangel's Staff
Not too expensive to upgrade since you already bought a Tear if you are playing suicide Karthus. By this time your Tear has seasoned a bit, and now buying this, along with your Deathcap passive, will massively increase your total AP.

Late Game

Void Staff
This item is necessary to pick up late game. More often than not, when the enemy sees your crazy damage, they will be buying some Magic Resistance to counter you. How to counter them countering you? Void Staff. It gives you percentage MPen, which, along with your Sorcerer's Shoes and your masteries and runes, will let you shred through ANYONE, whether a squishy with only base MRes or a tank with 5 Forces of Nature. Of course, this item also gives some nice AP.

Abyssal Mask is another possible option, but not as strong on Karthus, especially suicide Karthus, since even if you are not suiciding, there is a good chance you will die relatively quickly (which is fine because then you can deal more damage in Death Defied as well as ult), so the extra MR isn't too necessary. As Karthus, you need to deal as MUCH DAMAGE AS POSSIBLE, and Void Staff will help you do that -- Abyssal Mask won't. Especially if you are using your ult and not within 1000 range of everyone (highly likely), you will be dealing a lot less damage than you would be if you had Void Staff, even on targets that aren't building a lot of MR.



Mercury's Treads
I swap my Sorcerer's Shoes out for these late game if I am getting bursted down heavily by a lot of magic damage, or the multiple enemies are stacking a bunch of MR (see the Sorcerer's Shoes section). Since the sell value for Sorcerer's Shoes is 770, you are essentially only paying 430 gold for these boots, which are worth way more than that at this point in the game.

This swap is not in the main build since it is situational, but should be considered when you are getting bursted down quickly by the enemy AP(s), the enemy team has a lot of CC that is messing you up, and/or multiple enemies have over ~200 MR.



Zhonya's Hourglass
This item synergizes really well with Karthus's kit. Whether you are playing suicide Karthus or not, it essentially adds two seconds to your Death Defied, except that you can't cast anything during it, but you can still have your Defile going. Can be used as well to buy your team a few seconds to get over to you and clean up. Adds some nice armor, which helps when you are trying to kill the enemy AD/they are trying to kill you, and gives a good AP boost. Good finishing item for Karthus.



Elixir of Brilliance
This is the item to just keep spending your gold on if the game is STILL going and you have completed your item build. If you have all 6 item slots full, as you should if you are buying this item at the end, it will automatically be consumed for you. It gives some nice AP (over 50 with Rabadon's Deathcap at level 18) and CDR, which helps a load.

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Rylai's Crystal Scepter
Since all of your abilities are AoE (except single target Lay Waste), you will only be getting a 15% 1.5 second slow on all of them from this item. You already have a long ranged, long (wall length wise), long duration, AoE super slow. More often than not, buying this item would replace a more useful item on Karthus. However, if you find yourself getting bursted down multiple times before you are able to get in position on the enemy carries or in the middle of the enemy team with Death Defied, it might not be a bad idea to pick this item up. Just keep in mind that you would be replacing it with an item usually more useful on Karthus (the item you replace it with is up to your discretion).

Why no Rod of Ages AND Archangel's Staff in one build?

NOTE: THESE ITEMS CAN REPLACE ANY ITEM IN THE BUILD THAT YOU CHOOSE. THINK WISELY, HOWEVER, OF WHAT ITEM(S) YOU CHOOSE TO REPLACE WITH THESE SITUATIONAL ITEMS. IF YOU DO NOT NEED THESE ITEMS TO IMPROVE YOUR GAME AGAINST YOUR ENEMIES, DON'T BUY THEM.

Generally, however, if you end up actually getting to a full build, and you don't know which item you should drop for one of these, drop the Void Staff if they are mostly squishy (since it won't help too much), or the Zhonya's Hourglass if they are mostly tanky (since you need Void Staff to do more damage to them, but they won't do as much to you, so you can afford to have less defenses). Although you generally want all of the items in the main build if the game goes long enough to permit.


Quicksilver Sash
This item is good against enemies with a lot of CC (Kennen, Morgana, Lux, Vlad, etc) if you do not have Cleanse with you. It is cheap and has a cleanse-like active. It removes ALL debuffs from your character. Yes, this includes things that cleanse won't remove, such as damaging debuffs like Vlad's ulti, Malz's ulti, EVERY single debuff to you. Just use the active and Flash out of there (if running survivable Karthus). Keep in mind, if they CC/debuff you again right after you use the active, it won't have a reduced effect as it does with Cleanse. BE CAREFUL OF THAT. This item also gives a good amount of MRes, which is nice against these types of enemies.



Mercury's Treads
This item is also great against a team of CCers, just keep in mind it would supplement your Sorcerer's Shoes early game, so you will be dealing slightly less damage overall until you get your Void Staff. It might be worth it to have slightly less damage early game, however, if it means that you can get CC reduction against a team full of CC that could mess you up otherwise for the ENTIRE game.



Banshee's Veil
This item can be a lifesaver against certain enemies, such as enemies with CC like Morgana and Kennen, as well as enemies who have an "inevitable ultimate", such as Vlad's ultimate and Karthus's ultimate. It blocks a negative spell every 45 seconds. 'Nuff said. Getting in a sticky situation, your Banshee's might just save you enough time to get the hell outta there. This item also grants Health and mres, both helpful against these types of enemies. It also gives some mana, which doesn't hurt.



Mejai's Soulstealer
Nothing else at this price (let alone most other items) can provide this much AP. Plus, with the deathcap, not even including the bonus % AP masteries, the total 180 AP from this item with full stacks turns into 234 total AP. And getting stacks, especially with Karthus (click R to win), is super easy. Your abilities are so devastating early through late game that you are guaranteed to get a lot of kills and assists. And if you lose some, just get them back! Once you get full stacks, you get 15% CDR, which is helpful when you don't have blue buff or blue elixir on you.

However, this is NOT the most reliable item, use it at your own risk. If you are having an exceptionally good game (think: super stomp), try to pick it up early. If not, don't bother risking to lose an item slot and a lot of AP and some gold. It also adds a big "FOCUS ME" sign on top of your head, since the enemies will want to do all they can to prevent you from getting to 20 stacks. Be aware that this is a risky item to use, but will reap great rewards if you get a lot of stacks on it without dying.

If you want to take the time to check out my other guides, feel free to! Click the stamps to see the corresponding champion's guide!