Revive Karthus if he gets a takedown during death defied

I think Riot had some ideas for balancing it but they decided it wasn't a healthy mechanic to keep in his kit on a fundamental level.

There's no other champion in the game who poses a big AoE threat in teamfights AND can die in the middle of a team and continue damaging them. So he already has so much potential to punish you if you kill him at the wrong position/timing, that a revive on top of that would be too much even if it had a cooldown.

Anivia is 100% hp, and Aatrox can move. Reviving with 10% hp would be certain death for all but tank Karthus