Mortuary + Bolas's Citadel + Carrion Feeder + Willow Geist

Mortuary

Bolas's Citadel

Carrion Feeder

Willow Geist

How it works

1. Activate Carrion Feeder by sacrificing Willow Geist.
2. When Willow Geist dies, it and Mortuary triggers.
3. Resolve the Mortuary trigger, putting Willow Geist from your graveyard onto the top of your library.
4. Resolve the Willow Geist trigger, causing you to gain 1 life
5. Resolve the Carrion Feeder ability.
6. Cast Willow Geist from the top of your library by paying 1 life due to Bolas's Citadel.
7. Repeat.

This ingenious loop turns a self-sustaining sacrifice engine into an infinite storm and lifegain machine. You feed Willow Geist to Carrion Feeder, sending the Geist to the graveyard where Mortuary immediately intercepts it to place it right back on top of your library. From there, Bolas's Citadel lets you recast the creature straight from the library for a mere 1 life instead of its mana cost, while Willow Geist's own death trigger gives you that 1 life right back to keep the cycle going indefinitely.