I'm not saying that this is the only way to play Karthus, but if you don't play Karthus this way, you're probably a man who likes to pleasure other men sexually. Lol, but on a serious note, this (in my eyes) is the best way to play Karthus. This build focuses on Mana and Ability Power, and will make you a complete nuke. However, this build does offer very little in the way of survivability. I will provide a second build for those who are uncomfortable with the low amount of health in the first build. But, like I said, Karthus is best played as a nuke. With his global ultimate, a Pentakill with that much AP is almost guaranteed.

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Pros Huge Damage Output Global Ult Huge AoE 1000 Damage Q ~1 Second CD Great Mid if played correctly Looks Really Cool

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Cons Extremely Squishy Has no CC except Wall of Pain Countered by and Somewhat slow Early Game His ult is easily interrupted Respawn Problems due to

These provide a good amount of Magic Pen, which is vital to any AP Caster
These give a good early game Mana Boost, and also work quite well with Archangel's Staff
You could also take but I find these more helpful late game
These give a great deal of early game AP and can be a real difference maker in getting off to a good start with first blood.

The alternative being:

Again, Magic Pen is vital to any AP caster
Greater Seal of Vitality These give you that sustainability and health needed to be focused less or make you harder to kill.
It's either this or but again, I find these more helpful late game
That early game AP Boost you need to get yourself first blood.

Nuke Core Items

So, early game, obviously get , but late game, trade those in for another Archangel's Staff
The Archangel's Staffs are so good for this Karthus. For every staff you have, you get 400 MP. If you have 5 that's 2000 MP plus the 1000 MP unique passive. Then, the Passive stacks, so you get 3% per staff stacking up to 15% per staff. So, you take your MP which is roughly 4500 once the build is complete, and you take 15% of that which is 675, then you add 140 from Rabadon's Deathcap taking you to 815, then taking the passive from Rabadon's to a whopping total of 1060 AP just from items. The total from Items, Runes, and Masteries ends up being about 1350 AP. That means Requiem is hitting for about 1360, and Lay Waste is hitting for either 525 or 1050. If you have a spell that deals about 1000 Damage and only has a 1 second cooldown, you sir, are going to have a "good game."

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AP Off-tank Core Items

Now Boots you can either get Mercury's Treads or Sorcerer's Shoes depending on if you really want to go AP off-tank all the way or not. Rod of Ages gives both health and mana with a decent amount of Ability Power. Rylai's Crystal Scepter gives both health and AP and gives you another slow. This works really well with Wall of Pain and Lay Waste because they can never escape and they have lowered Magic Resistance. Abyssal Mask gives AP and magic resist and has a passive that works well with Wall of Pain Zhonya's Hourglass gives huge armor and AP and has an active that can be used well against enemy's such as Karthus, Vladimir, Cho'Gath, Galio, Caitlyn, Katarina, and Garen, among others. The Rabadon's Deathcap is pretty Self-Explanatory, it just gives a massive AP boost. Even with this build, you're still going to be hitting very hard, and have much more survivability, but I think it's way more fun to play like it's August 6, 1945 in Hiroshima. (wow that was horrible, I'm sorry)

There's pretty much only one way to do this any way you look at it. It's crucial to go in this order.

Requiem> Lay Waste> Defile> Wall of Pain

Death Defied It's Karthus's Passive, and allows him to remain a threat even when dead, and many enemies will ignore him and give you free kills with your AoE and Q.

[Lay Waste] Huge damage spell with about 1 second of cooldown. It can land big hits late game. About 1000 actually.

Wall of Pain A slow and lowers Magic Resistance. Karthus's only CC. use it wisely

Defile Karthus's AoE: Does a lot of damage, and restores mana passively.

Requiem An amazing global ult that finishes enemies like a champ. Make sure you follow my advice and go 21/0/9 masteries, because hits harder with Executioner

There are two main summoner spells for mages, unless they have a built in escape which is not very common. The exceptions being LeBlanc, Ahri, Ezreal - depending on how you build him - , Fiddlesticks, Akali, and Fizz Okay, so maybe it is a little more common then I thought, but regardless, you should go with Flash because it can either help you chase or escape, which are two crucial parts of both laning and teamfights. The other spell you should take is Ignite, which can give you that tiny bit of extra damage you needed to finish off the one that got away.

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Ghost is a good escape and pursuit tool.
Clarity is good for replenishing mana.
Heal is good for healing
Teleport can save a turret.
Cleanse is good for escaping from a team with a lot of CC.

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Surge: You're an AP caster, why would you use Surge?
Promote: There's much better options
Clairvoyance: There's much better options
Smite: Get out of the Jungle and back to mid Karthus
Exhaust: Meh, you usually lane against an AP mid, plus there's no way to know for sure if a ranged carry will take mid.

Karthus is an excellent farmer, once he has enough AP, a combination of Defile and Lay Waste destroys minion waves like no other.

With Karthus's amazing ult, Requiem, you are such a threat to the entire enemy team. However, you need to make sure you and your teammates have good communication, otherwise, it will be difficult to know when to cast your ult at times. Also, in team fights, stay back and wreak havoc with Lay Waste, lay down Wall of Pain to both lower resistance and allow escape, and use Requiem at the right times. If done correctly:

PENTAKILL

Please comment and if you found this guide useful, upvote. I did spend A LOT of time on this, and it would be really nice to see some good feedback. However, if there is any reason that you do find it necessary to downvote, please tell me why so I can fix my mistakes.

Also, keep in mind that Karthus can be very hard to get used to as he is a difficult champ, and will take many games to master.

Happy Killing,
-Dumb Bum