Mortuary + Gray Merchant of Asphodel + Thermopod + Bolas's Citadel

Mortuary

Gray Merchant of Asphodel

Thermopod

Bolas's Citadel

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 Thermopod by sacrificing Gray Merchant.
4. When Gray Merchant dies, Mortuary triggers, putting Gray Merchant from your graveyard onto the top of your library.
5. Repeat.

This clever loop relies on Bolas's Citadel to continually cast Gray Merchant of Asphodel directly from the top of your library by paying life rather than mana, a cost easily offset by the Merchant's own life-draining entry ability. Once the Gray Merchant has delivered its devotion-fueled drain, you feed it to Thermopod to generate red mana and send it to the graveyard. That's where Mortuary steps in, instantly intercepting the dead creature and placing it right back on top of your library so Bolas's Citadel can play it all over again.