I'm working on a new guide for Karthus at the moment, but here's my current semi-carry DPS build:
Tear of the Goddess
Rod of Ages
Situational item: Abyssal Scepter/Deathfire Grasp/Void Staff
I used to go Clarity/Flash, but I've now runed enough mana regen that I can go Flash/Teleport or Flash/Exhaust. I also rune for 10 spell penetration.
Masteries are 9 points in Offense for the 15% spell pen, rest in utility, grab movement speed, mana regen, xp boost, etc.
This build doesn't offer much in survivability, but the idea is you won't be getting targetted since you can harass from a huge range with Lay Waste, and Defile is popped once everyone has already committed in a team fight.