Rites of the Blood God

At the beginning of your end step, sacrifice a creature. If you can't, you lose the game.
Creature spells you cast have bloodthirst X.
(This creature enters the battlefield with X +1/+1 counters on it, where X is the damage dealt to your opponents this turn.)
"His demands were simple, and the punishment severe, so we obeyed. What's one life compared to the power of a god?"