Mortuary + Bolas's Citadel + Carrion Feeder + Vengeful Bloodwitch

Mortuary

Bolas's Citadel

Carrion Feeder

Vengeful Bloodwitch

How it works

1. Activate Carrion Feeder by sacrificing it.
2. When Carrion Feeder dies, Mortuary and Vengeful Bloodwitch trigger.
3. Resolve the Mortuary trigger, putting Carrion Feeder from your graveyard onto the top of your library.
4. Resolve the Vengeful Bloodwitch trigger, causing you to gain 1 life and target opponent to lose 1 life.
5. Resolve the Carrion Feeder ability
6. Cast Carrion Feeder from the top of your library by paying 1 life.
7. Repeat.

This loop turns a modest life total into a lethal engine by leveraging Bolas's Citadel, which allows you to continually cast a creature straight from the top of your library by paying life instead of mana. By feeding Carrion Feeder to its own ability, you trigger Mortuary to neatly recycle the sacrificed creature right back onto the top of your library where the Citadel can reach it again. Meanwhile, Vengeful Bloodwitch turns every single iteration of this sacrifice cycle into a net-zero life transaction for you while draining your opponent dry.