Mortuary + Umbral Collar Zealot + Bolas's Citadel + Bontu's Monument

Mortuary

Umbral Collar Zealot

Bolas's Citadel

Bontu's Monument

How it works

1. Activate Umbral Collar Zealot by sacrificing a creature with mana value 1 or less.
2. When the creature dies, Mortuary triggers, putting the creature from your graveyard onto the top of your library.
3. Resolve the Umbral Collar Zealot ability.
4. Cast the creature from the top of your library by paying 1 life.
5. Bontu's Monument triggers, causing you to gain 1 life and each opponent to lose 1 life.
6. Repeat.

This clever engine loops a creature with a mana value of 1 or less straight from the battlefield back into your hands via your library, fueled entirely by life rather than mana. Bolas's Citadel lets you cast that creature directly off the top of your library for a mere 1 life, and Bontu's Monument happily mitigates that cost by draining your opponents and gaining you life whenever you cast it. Meanwhile, Umbral Collar Zealot provides the sacrifice outlet and surveil utility to churn the piece back down, while Mortuary catches the creature the moment it dies and places it right back on top of your library to restart the cycle.