Mortuary + Gray Merchant of Asphodel + Umbral Collar Zealot + Bolas's Citadel

Mortuary

Gray Merchant of Asphodel

Umbral Collar Zealot

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 Umbral Collar Zealot 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 Umbral Collar Zealot ability.
6. Repeat.

This infinite loop leverages Bolas's Citadel to cast Gray Merchant of Asphodel directly from the top of your library for life instead of mana, draining your opponents and padding your life total in the process. Once the Merchant has done its work, Umbral Collar Zealot steps in to sacrifice it, which immediately triggers Mortuary to place the Merchant right back on top of your library where it can be cast all over again.