Mortuary + Zulaport Cutthroat + Bile Urchin + Bolas's Citadel

Mortuary

Zulaport Cutthroat

Bile Urchin

Bolas's Citadel

How it works

1. Activate Bile Urchin by sacrificing it.
2. When Bile Urchin dies, Mortuary and Zulaport Cutthroat trigger.
3. Resolve the Mortuary trigger, putting Bile Urchin from your graveyard onto the top of your library.
4. Resolve the Zulaport Cutthroat trigger, causing you to gain 1 life and each opponent to lose 1 life.
5. Resolve the Bile Urchin ability.
6. Cast Bile Urchin from the top of your library by paying 1 life.
7. Repeat.

This ingenious loop turns Bile Urchin into a self-sustaining engine by trading your life total directly for endless damage. Bolas's Citadel makes casting the Urchin from the top of your library trivial by swapping mana costs for life payments, which is easily offset because Zulaport Cutthroat drains your opponents while gaining you life on every single iteration. Finally, Mortuary quietly catches the dying Urchin and places it right back on top of your library, seamlessly resetting the entire sequence for another trip through the graveyard.