This is hands down the best build I've played with Karthus. I got is from watching a member of CLG or SK in the Dreamhack qualifications in 2011.

I'm not hear to teach you how to play as Karthus, you can do that through practice and reading the other guys who have time on their hands to make builds for amateur gamers.

Let me know what you think of the build setup, I know a lot of people may not understand why I used exhaust and flash, but when you realize that no one engages you due to the fact that your AoE for E and Q plus the spell vamp heals you faster than they can damage you.

exhaust is for those dumb people that try to engage you then run away, while your right behind them Q'ing the **** out of them.

Let me know what you think.