Mortuary + Garruk's Horde + Hangarback Walker

Mortuary

Garruk's Horde

Hangarback Walker

How it works

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

This loop turns the zero-mana casting of Hangarback Walker into an endless cycle of self-sacrifice, entirely fueled by Mortuary's triggered ability placing the creature straight back onto your library. Once Garruk's Horde reveals the top of your library and grants you permission to cast creature spells from it, you can replay the Walker for free over and over again. Because Hangarback Walker enters with zero toughness and immediately hits the graveyard, you seamlessly generate infinite creature enters-the-battlefield triggers, leaves-the-battlefield triggers, deaths, and storm count without ever needing to spend a single mana.