Ashnod's Altar + Thran Vigil + Gravelgill Axeshark

Ashnod's Altar

Thran Vigil

Gravelgill Axeshark

How it works

1. Activate Ashnod's Altar by sacrificing Gravelgill Axeshark.
2. When Gravelgill Axeshark dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Thran Vigil triggers, putting a +1/+1 counter on Gravelgill Axeshark.
4. Repeat.

This infinite loop relies on a clever counter-neutralizing interaction centered around Gravelgill Axeshark and its persist ability. When you sacrifice the Axeshark to Ashnod's Altar, it returns to the battlefield with a -1/-1 counter, but the moment it leaves your graveyard, Thran Vigil triggers to place a +1/+1 counter on it; state-based actions then cause the +1/+1 and -1/+1 counters to annihilate each other, resetting the Axeshark so persist can trigger all over again. Meanwhile, the Altar turns every iteration of this cycle into a steady stream of colorless mana and death triggers, letting you fuel massive spells or drain your opponents endlessly.