Starting item, Meki pendant. No pots. A good Karthus doesn't need pots. You shouldn't be trading early, and you shouldn't run out of mana once your E is rolling. Upgrade asap to Tear. Get boots. Once tear can be upgraded to AA, do so. Somewhere before AA is built you'll have enough for your sorc boots, get em. Then the rest of the game just get the following items. Rabs, Void Staff, last two are situational. Zhonyas, Rylais, etc. .Just depends on who you're facing and how things are going. The key items every Karth should have though, are AA, Rabs, and Sorc shoes. I get a void staff only if the other team is building MR, otherwise I let the shoes be enough to cut through.