"The time for subtlety is over, darling."

Yahenni, Undying Partisan

Haste
Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni.
Sacrifice another creature: Yahenni gains indestructible until end of turn.

The List

The Lost Caverns of Ixalan Commander

Haste
Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni, Undying Partisan.
Sacrifice another creature: Yahenni gains indestructible until end of turn.

Commander Masters

Double Masters 2022

If Yahenni and a creature an opponent controls die simultaneously (perhaps because they fought or were in combat together), Yahenni won't be on the battlefield as its triggered ability resolves. It can't be saved by the +1/+1 counter that would have been put on it.