Toph, the First Metalbender + Biotransference + Yahenni, Undying Partisan

Toph, the First Metalbender

Biotransference

Yahenni, Undying Partisan

How it works

1. Cast the earthbending creature by paying its mana cost.
2. When you cast that spell, you lose 1 life and create a 2/2 artifact creature token.
3. The earthbending creature enters as an artifact land creature due to Biotransference and Toph.
4. When that creature enters, it earthbends, causing itself to become a land creature with haste, putting at least one +1/+1 counter on itself, and creating a delayed triggered ability.
5. Activate Yahenni by sacrificing that creature.
6. When that creature dies, the delayed ability from step 4 triggers, causing it to return to the battlefield tapped as an artifact land creature.
7. Repeat from step 3 any number of times.
8. Resolve all activated abilities from Yahenni.

This ingenious loop relies on a brilliant alignment of types: thanks to Biotransference, all your creature spells are artifacts, which triggers its life-loss and token-creation ability whenever you cast one. Meanwhile, Toph, the First Metalbender ensures your nontoken artifacts are also lands, allowing them to benefit directly from earthbending mechanics when they hit the battlefield. Once your earthbent creature enters, you can freely sacrifice it to Yahenni, Undying Partisan to protect Yahenni and trigger the creature's own earthbending return clause, creating an endless cycle of sacrifice, death, and return.