Mortuary + Gray Merchant of Asphodel + Blasting Station + Bolas's Citadel

Mortuary

Gray Merchant of Asphodel

Blasting Station

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 and Blasting Station trigger.
3. Resolve the Blasting Station trigger, untapping it.
4. Resolve the Gray Merchant trigger, 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.
5. Activate Blasting Station by tapping it and sacrificing Gray Merchant.
6. When Gray Merchant dies, Mortuary triggers, putting Gray Merchant from your graveyard onto the top of your library.
7. Resolve the Blasting Station ability.
8. Repeat.

This loop turns the top of your library into an endless armory, fueled directly by life rather than mana thanks to Bolas's Citadel. You simply cast Gray Merchant of Asphodel over and over again from the top of your library, paying its life cost while Mortuary quietly catches it upon death to tuck it right back where it started. Every time the Merchant re-enters, Blasting Station untaps itself so you can sacrifice the creature anew, draining your opponents' life totals through a combination of the Merchant's devotion trigger and the Station's relentless artillery.