Mortuary + Extruder + Memnite + Bolas's Citadel

Mortuary

Extruder

Memnite

Bolas's Citadel

How it works

1. Activate Extruder by sacrificing Memnite
2. When Memnite dies, Mortuary triggers, putting Memnite from your graveyard onto the top of your library.
3. Resolve the Extruder ability.
4. Cast Memnite from the top of your library by paying 0 life.
5. Repeat.

This clever engine loops for free by turning Bolas's Citadel's alternative life-payment cost into a continuous recycling plant, letting you cast Memnite for zero life right off the top of your library. Whenever Extruder sacrifices Memnite for a +1/+1 counter, Mortuary immediately catches the falling automaton and places it right back on top of your library where Bolas's Citadel can see it again. Because Memnite costs zero mana, you never actually lose life to Bolas's Citadel's ability, allowing you to build an infinite storm count and trigger endless deaths, enters-the-battlefield effects, and creature sacrifices.