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.
First of all: why tank?
Karthus have already powerfull spells even if he doesnt have so much ap and making him as a tank make a huge chaos into an enemy team. You will be targeted first and enemy teams needs a lot of team to kill you (not that you have a lot of mana for defile so you already dealt propably a lot of damage to enemy's team - if they are not dead yet)
-Play mid and farm a lot with Lay Waste and Defile (Playing Bottom or Top might makes your game more difficult due gold)
-Get Buffs if possible
-Dont forget that your not indestructible, youre perfect as second tank but it is possible to play well as main tank.
-Use your Cleanse when you are getting focused by whole team
My 1st 4 games with this build: