Mortuary + Bolas's Citadel + Marketback Walker

Mortuary

Bolas's Citadel

Marketback Walker

How it works

1. Cast Marketback Walker either from your hand by paying 0 or from the top of your library by paying 0 life.
2. Marketback Walker enters and dies as a state-based action due to having 0 toughness.
3. Mortuary triggers, putting Marketback Walker from your graveyard onto the top of your library.
4. Repeat.

This ingenious engine turns a creature with a fatal flaw into an eternal loop by bending the rules of where your cards can live. You play Marketback Walker for free from the top of your library thanks to Bolas's Citadel, though its zero toughness immediately sends it crashing to the graveyard as a state-based action. Instantly, Mortuary catches it on the way down and redirects it right back to the top of your library, setting you up to cast it all over again without ever needing actual mana.