Mortuary + Bolas's Citadel + Carrion Feeder + Susurian Voidborn

Mortuary

Bolas's Citadel

Carrion Feeder

Susurian Voidborn

How it works

1. Activate Carrion Feeder by sacrificing it.
2. When Carrion Feeder dies, Mortuary and Susurian Voidborn trigger.
3. Resolve the Mortuary trigger, putting Carrion Feeder from your graveyard onto the top of your library.
4. Resolve the Susurian Voidborn 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 looping engine relies on Bolas's Citadel to let you cast creatures directly from the top of your library by paying life instead of mana. When you sacrifice Carrion Feeder to its own activation, Mortuary instantly catches it on the way to the graveyard and stacks it right back onto the top of your library for the Citadel to grab again. Meanwhile, Susurian Voidborn watches the sacrifice happen and drains your opponent's life total on every single iteration to offset the life you're spending.