This is my first build... This is how I play Karthus and I always dominate mid and win teamfights. In a teamfight, everyone focuses Karthus and you usually die, but you still help your team to win that teamfight because of his awesome passive - Death Defied and other spells. This guide is on Karthus on mid and focuses on his awesome ultimate - Requiem. Before you decide to give this build a thumb up or down, read the whole build.

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Very High Damage
Small Cooldowns
Lots of Mana
Awesome Harrass


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Focused in Teamfights
Kinda Squishee
Mana Hungry

I choose a lot of flat Ability Power and Magic Penetration and I think it's better than other things usually per level. I prefer to have a good early game, so I can farm.


This is a perfect rune for all mages and especially for Karthus. It gives you a lot of magic damage and I think there is no reason to have a different rune.


This is also a very good rune that gives you a good early game but can be replaced by Greater seal of vitality that gives you better late game. It is your choice of what do you need but I prefer the flat AP.

VS.
I prefer flat AP rune, because it gives you a big advantage in the beginning of 9 AP. The ...over level AP gives you just about 27 AP on level 18 where I usually even don't get, but it's just a matter of what you need and play him like.


This is again a rune which is not replaceable with any other good rune and is good for all mages.

spell vamp
Gives you cooldown reduction
And many other things...

Offense:
Gives you ability power
Gives you cooldown reduction
Gives you magic penetration

I choose a lot of cooldown reduction to have a low cooldown on your spells especially Requiem and Flash and some other things like Spell Vamp and Ability Power for maximum damage.

You can buy this instead of Doran's Ring for greater mobility and easily avoid enemy harass


Makes you not so squishee and not so mana hungry. It is a perfect item in the begining and I really recommend it.


This is also a perfect item for Karthus. It gives him the things he needs... Ability Power, Mana Regen and Cooldown Reduction. I also recommend this item.


This item could be replaced by other items, but when you have lots of mana or you need it, it gives you a lot of it and Ability Power.


This is I think an item that really depends on the situation. I always build it last and only when the other team has some tank. Otherwise is much more useful in combination with Defile.

You dont have to play it like this. It really depends on the situation. If you want to use your ultimate often and harass as you can, this is the perfect build, but if there are no tanks and you die very often, buy Zhonya's Hourglass instead of Deathfire Grasp and Void Staff is very useful also.

I max this anytime I can and I think it is the best option. If you can hit a single target with it, it deals tons of damage and is also very useful to farm minions in combination with Defile


This is more useful in late game. It gives you a long advantage of speed and damage, but it is good when you take it early and leave it there until you can't take anything else.


This is very important for farming and or teamfights. If you feel like you have problems with mana, max this, because of it's awesome passive but I never had such problems with mana and maxing Lay Waste is for me more useful.


I think I shouldn't say much about this. As almost every other ultimate, try to max it as you can (levels 6,11,16) and use it on these levels because it is strongest on lvl. 6,11 or 16.

On level 1, just stay under the turret until minions spawn with Doran's Ring and Lay Waste. When minions arrive, just farm them (more details in "farming") and try to harass your opponent with Lay Waste and try to hit only him and not any other target. After you have Defile and Wall of Pain you can start playing more offensively. When he is not behind his minions, put Wall of Pain in front of him and try to hit him with Lay Waste as many times as you can. If he is very close to you, you can also turn Defile on but beware of your mana. If you have low health or mana, recall. Buy things you can and if you need a few Health Potion. Sometimes go for the blue buff or dragon.

In a teamfight, you should go directly in the opposite team, turn Defile on, put the Wall of Pain there and if you have Zhonya's Hourglass use it. Then try to harass the most squishee champions with your Lay Waste. Probably they focused you and you died, but no problem. Let Defile on because it cost no mana and use Requiem if necessary and try to hit a few enemies with Lay Waste or Wall of Pain. Your team should be able to win the teamfight and you will get at least a double kill.
In teamfights you are strongest on levels 6,11 and 16 because your ultimate is strongest there.

In early game, try last hitting the minions with your basic attack or Lay Waste but be sure you are not wasting too much mana. When you have Defile mana should't be problem and it usually gives you more mana than Lay Waste costs. When you have Defile and Lay Waste maxed, kill the minions with Defile and one or two Lay Waste and the whole minion wave should be dead.

This helps you to chase down, or to escape your enemies (try to flash over walls). It is very good for Karthus but can be replaced.


When you are on mid, Teleport really helps you to push and farm there or to safe or gank anywhere on the map.


Personally, I don't have any problems with mana even with Doran's Ring. If you have problems with it, this really helps you and also you will be able to stay and farm on mid even longer.


i don't think this is necessary for Karthus. When you want to run away, just use Wall of Pain or Flash and the same for chasing your enemies. I don't recommend it.


This is good in combination with Clarity because it helps you to stay longer and farm. Also it is good if you think, you will have a hard time on mid.

Other spells I think are just non-sense and not useful for Karthus on mid, but maybe you have something different and it works for you, but not for me.

This is just how I play Karthus. It doesn't have to work for you, but it works for me. I never had negative score and got at least a double kill in every game. Good luck playing Karthus and I wish you a lot of PENTAKILLS !!!