Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Let's be honest this is the only way to build a good Karthus, you're a glass cannon, that's what you are made to be. Your Defile is as good if not better then Fiddlestick's Ult, Your is one of the best if not the best ult. Wall of pain is one of the best CC skills in the game, and you can solo 1v1 a dps with Lay waste. Karthus is a god, now let's build him as such.
This guide is still in progress.
Pros / Cons
-You can 1v1 as well as a DPS
-Your ult and passive guarantees you kills
-You might actually make the enemy rage quit
-You will get focused
-Veigar = GG