Karthus is a very dangerous champion to push against.
This build focuses on being a one man army so to speak.
Devastatingly powerful with low cooldowns, he even defies death which with a certain tactic along with this item build can destroy entire teams.
Im no professional and this is not a guide, it's simply a build and tactic on how to use it efficiently.

1126 ABILITY POWER!!! be sure not to be the ****** who looks at the lvl 18 stats and assumes they account for all items effects.

During team fights Karthus can do one of 2 effective tactics:

1. He can Flash into the middle of the enemy team and activate Defile then using lichbane to deal 1100 md to the enemy carry before dieing. Use Deathfire Grasp against the enemy tank to take out 68% of his hp. Then Lay Waste as much as you can to kill the remaining champions (do not focus on enemy tank). Use Wall of Pain on your enemy's escape route and finally Requiem before your time from Death Defied expires.

2. Same as 1 but instead of using Flash to get into the middle of the enemies you can run in from the brush from behind using Ghost to get as close as you can to the center of them.

This maximizes the use of all his abilities and usually results in 3-5 kills before Death Defied expires. The help you would normally get from your team should ensure an ace as long as they can run fast enough to chase the survivors as they run away.
Karthus should be the only one dieing in team fights (it's what he was built to do, note: no spell vamp).