Mortuary + Daxos, Blessed by the Sun + Bile Urchin + Bolas's Citadel

Mortuary

Daxos, Blessed by the Sun

Bile Urchin

Bolas's Citadel

How it works

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

This brilliant loop turns the life-payment mechanic of Bolas's Citadel into a self-sustaining engine by utilizing Bile Urchin as both your sacrificial outlet and your win condition. Whenever you sacrifice the Urchin, Mortuary steps in to intercept it from the graveyard and stack it right back onto the top of your library, making it immediately available to be cast again off the Citadel's top-deck permission. Meanwhile, Daxos, Blessed by the Sun pads your life total with a gain of 1 life for every single entry and exit, completely neutralizing the life cost of recasting the Urchin and netting you infinite life loss for your opponents along the way.