Karthus is my main champ, i dont understand why they put him as a hard champ to play, personally, i think he was easy to master compared to say, Nid.

Anyways, i always modify my build to every match depending on enemyteam and the items they choose to buy. But overall this is how i do it, and ive had very good luck with it, in ranked i havent lost a single match going mid with karthus with this build.

It is important to know that you NEVER initiate with karthus, always stay behind the rest of the team and whenever there is a teamfight, you dont wait until the fight is over using your ulti to get a kill, you use your ulti right after the teamfight starts, and 10 out of 10 times, your team will win the fight.

karthus is an AP carry, play him as such, early to mid game, steal kills if you have to with your ulti to get the soulstealer up, once you hit mid to late game, you stay BEHIND your team and let them initiate... never jungle alone always walk with others to awoid ganking. Use lay-waste and wall of pain to scout bushes before you enter...

karthus it probably the most IMBA or OP champ in the game, combine him with some skilled tanks and he will carry the team to victory!

-happy hunting :)