Quote from _Ai_What's a good build for Kat? Right now I'm just focusing on three main items: Deathfire Grasp, Rabbadon's, and Liandrys.
Occasionally buy Sorcerer Shoes. I start out with the Amplifying Tome.
One thing, don't upgrade your Haunting Guise until its your last item to spend money on (or they're stacking health and no MR). I go Void Staff after DFG and Rabadoobas, but its also important to get defense so you can last in fights until its time to drop the ult. So Rylai's and Abysall Scepter (only if they have lots of AP) are good for that.