Not Updated For Current Season
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: