In the late game this build is great for defending home nexus when you team start to push a lane, you can take out waves of minions very fast and therefore getting you alot of gold in the process.

When you are playing Karthus and you have team battles always remember to use your slow wall, try to take up as much of a lane as you can with it. when you use you wall remember to get right up and close with the enemy champion and hit defile, the whole time you are in the battle you should be using lay waste where ever you can. this will drain alot of you mana quickly so be aware before a team battle that clarity is up.

With Karthus it is Key to keep other champions away from you and your turrets by using lay waste as much as you can, it will drain your mana fairly fast but that is why you are building mana off the start.

The biggest thing that i cant stress enough is that in mid-late game when people are in there own lanes pay attention to the whole map. be prepared to use your ult to get a kill that is escaping or to lower someones heath in order to help a team mate.