The One Who Torments

Haste, first strike, double strike
All creatures your opponent controls get -4 every turn. Once all their creatures are in the graveyard, the damage transfers to the player. But to unlock this ability, you need to have The One Who Kills in the battlefield or graveyard.
If The One Who Torments dies/is exiled, you may pay {9} to get it back from the graveyard/exile.
Kill them.