Mortuary + Umbral Collar Zealot + Bolas's Citadel + Willow Geist

Mortuary

Umbral Collar Zealot

Bolas's Citadel

Willow Geist

How it works

1. Activate Umbral Collar Zealot 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 Umbral Collar Zealot ability.
6. Cast Willow Geist from the top of your library by paying 1 life due to Bolas's Citadel.
7. Repeat.

This clever loop turns your library into a self-sustaining engine by turning life into mana, utilizing Bolas's Citadel to cast Willow Geist directly from the top of your deck for just a single point of life. When Umbral Collar Zealot sacrifices the Geist to fuel its ability, Mortuary immediately intercepts the creature as it hits the graveyard, neatly tucking it right back onto the top of your library where Bolas's Citadel can grab it again. Meanwhile, Willow Geist itself benefits from this constant motion by triggering its own lifegain ability every time it leaves the graveyard, easily offsetting the life you pay to recast it and keeping your loop going indefinitely.