Mairsil, the Pretender + Endling + Cauldron of Souls + Blood Pet + Fervor

Mairsil, the Pretender

Endling

Cauldron of Souls

Blood Pet

Fervor

How it works

1. Activate Mairsil using Cauldron of Souls' ability by tapping it, giving itself persist until end of turn.
2. Activate Mairsil using Blood Pet's ability by sacrificing it, adding .
3. When Mairsil dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
4. Activate Mairsil using Endling's third ability by paying , giving Mairsil undying until end of turn.
5. Activate Mairsil using Blood Pet's ability by sacrificing it, adding .
6. When Mairsil dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
7. Repeat.

This loop transforms Mairsil, the Pretender into a self-sustaining engine by utilizing cage counters to accumulate a powerful suite of activated abilities from exile. While Blood Pet provides the black mana to fuel the process and Fervor ensures Mairsil can act immediately upon returning, the true magic lies in alternating between Cauldron of Souls and Endling. By granting Mairsil persist and undying alternately, every sacrifice cancels out the opposing counter type left behind by the previous resurrection, allowing you to loop endlessly for infinite mana, death triggers, and enters-the-battlefield effects.