Hello this is my second build in mobafire (Tryndamere was the first) I decided to create a build about Karthus (remeber the guy who makes pentakill even after he died yeah its him). Anyway since Karthus is usually made very squishy I made a build where even ad carry will think twice before charging straight at you.

For runes I choose magic penetration because its usesful against targets with magic resistance.
Armor seals becasue it helps to survive ad carry (in case if one is sent in the middle) also it helps against ad jungler (early game anyway).
Flat ap glypths for early game ability power because if you start with sapphire crystal your ap will be very small and you wont do anything in early game.
Flat ap quintessences (for the same reason I took ap glypths).

22/0/8 are masteries I usually take for any mana ap carry (except Ryze). If you make different masteries remember to spend atleast 21 point in offense.

With this item build you wont have the insane ability power but you wont be killed that easly either if you want alot of ap then buy 5 archangels staffs and rabadons deathcap. Buy Rod of Ages early so you can get those stacks early and always buy at least one Tear of Goddes for mana and mana regeneration. Rylais is a great item to make shure your enemies wont escape.

Always take lay waste at level 1. When you reach level 2 I recommend taking wall of pain and at level 3 lay waste or defile. I always max lay waste first and defile second but before maxing defile I always upgrade wall of pain to level 3.

Karthus is very powerful and usesful even after he is dead remember that lay waste will be your main damage ability and wall of pain supportive (to stop the enemy from escaping). Remember not to spam your ultimate pointlssly or right after you have died (unless you think that you can get a kill like that). You should ask your team-mates to inform you about enemy champions with very low hp or keep an eye out for them yourself Karthus ultimate is a good kill stealing tool (but I usually try not to killsteal unless I buy Mejais)

Flash is a very good escape as well as engage tool. Exhaust is very good on Karthus expecially when you have Rylais becasue your enemy cant escape you that easly and if you get attacked by ad champion you will mostle be able to escape him or even kill him.

Karthus is one of few ap champions who can engage team fights (expeciall after you have bought Zhonyas and you actuall USE its active ability) Karthus is very good for killing enemy or enemies wich have escaped team fights with barely any hp also your wall of pain can help chasing some enemy champions or saving your team (but if you are evil you wont). Remember if you die then die in the middle of enemies (becasue you defile is automaticlly activated when you die).

I hope that I have given some usesful hints about how to play Karthus better and dont mind all those players who keep crying after you kill them ''Karthus noob only press R''.