Mortuary + Bolas's Citadel + Willow Geist + Phantom Train

Mortuary

Bolas's Citadel

Willow Geist

Phantom Train

How it works

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

This loop creates a self-sustaining engine by turning the top of your library into an endless armory, fueled by Bolas's Citadel allowing you to cast spells using life instead of mana. When you feed Willow Geist to Phantom Train as a sacrifice, Mortuary steps in to immediately catch it from the graveyard and place it right back on top of your library. As Willow Geist leaves the graveyard, its own ability grows it larger while its death trigger gives you back the life you spent, offsetting the cost of casting it again with Bolas's Citadel.