Mortuary + Bolas's Citadel + Carrion Feeder + Popular Egotist

Mortuary

Bolas's Citadel

Carrion Feeder

Popular Egotist

How it works

1. Activate Carrion Feeder by sacrificing it.
2. When Carrion Feeder dies, Mortuary and Popular Egotist trigger.
3. Resolve the Mortuary trigger, putting Carrion Feeder from your graveyard onto the top of your library.
4. Resolve the Popular Egotist 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 elegant loop turns a humble Carrion Feeder into an engine of endless life drain, fueled entirely by the life-payment alternative of Bolas's Citadel. Whenever you sacrifice the Feeder, Mortuary intercepts it on its way to the graveyard to stack it right back onto the top of your library for the Citadel to cast again. Meanwhile, Popular Egotist watches the sacrifices happen, converting every single repetition into a life-draining trigger that outpaces the 1 life you pay to recast your creature.