Mortuary + Gray Merchant of Asphodel + Bolas's Citadel + Carrion Feeder

Mortuary

Gray Merchant of Asphodel

Bolas's Citadel

Carrion Feeder

How it works

1. Cast Gray Merchant either from your hand by paying 3 or from the top of your library by paying 5 life.
2. When Gray Merchant enters, it triggers, causing each opponent to lose life equal to your devotion to black and causing you to gain life equal to the life lost this way.
3. Activate Carrion Feeder by sacrificing Gray Merchant.
4. When Gray Merchant dies, Mortuary triggers, putting Gray Merchant from your graveyard onto the top of your library.
5. Resolve the Carrion Feeder ability.
6. Repeat.

This looping masterpiece turns your library into a repeating engine fueled entirely by life rather than mana. With Bolas's Citadel letting you play Gray Merchant of Asphodel directly from the top of your library for life instead of mana, you can cast the Merchant over and over again while offsetting the life payments with its own devotion-based lifegain. Whenever the Gray Merchant enters, you drain your opponents dry, after which Carrion Feeder steps in to sacrifice the Merchant to a growing heap of +1/+1 counters. Finally, Mortuary ensures the creature never stays in the graveyard long, instantly sliding it right back to the top of your library so Bolas's Citadel can play it once more.