Ashnod's Altar + Fiend Hunter + Metamorphosis Fanatic

Ashnod's Altar

Fiend Hunter

Metamorphosis Fanatic

How it works

1. Cast Fiend Hunter by paying its mana cost.
2. When Fiend Hunter enters, its first ability triggers, exiling Metamorphosis Fanatic.
3. Activate Ashnod's Altar by sacrificing Fiend Hunter.
4. When Fiend Hunter dies, its second ability triggers, returning Metamorphosis Fanatic from exile to the battlefield.
5. When Metamorphosis Fanatic enters, it triggers, returning Fiend Hunter from your graveyard to the battlefield.
6. When Fiend Hunter enters, its first ability triggers, exiling Metamorphosis Fanatic.
7. Repeat from step 3.

This loop creates an infinite sacrifice engine by leveraging Fiend Hunter's two-part ETB and LTB triggers to repeatedly cycle Metamorphosis Fanatic between exile and the battlefield. Whenever Fiend Hunter enters, it tucks away Metamorphosis Fanatic, only to release it when Ashnod's Altar sacrifices Fiend Hunter for a pair of colorless mana. The returning Metamorphosis Fanatic immediately triggers its own ability to fish Fiend Hunter right back out of the graveyard, restarting the entire cycle with mana to spare.