I must be missing something. Can someone explain prz?

Zombie Karthus
Look it up

what you are missing is teleport.

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its revive tele karthus that is op

You rambo into a teamfight, do as much damage as you can, die, ULT, revive, then use revive speed boost or teleport to jump back to whatever remains of the fight and clean it up.

Used to work with Pantheon and TF, as well. Still could, I guess, if the fight isn't too far from base.

Ohhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh hhhhh.

TY everyone =D

You become the guy that can do both initiation and clean up.

With Revive and Teleport, you can die in a teamfight, utilize the full 7 seconds to attack with your Q, then die, revive instantly, ult and teleport back in if there's anyone alive.

Of course, you become laughably easy to gank in lane.