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

Mairsil, the Pretender

Endling

Cauldron of Souls

Blood Vassal

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 Vassal'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 Vassal'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 elegant loop transforms Mairsil, the Pretender into a self-sustaining engine by cage-countering the right tools from your graveyard or hand. Fervor ensures Mairsil is ready to act immediately, while Cauldron of Souls and Endling provide the alternating persist and undying abilities that keep trading -1/-1 and +1/+1 counters to cheat Mairsil back from the graveyard every time it dies. Along the way, Blood Vassal provides the sacrificial outlet that funds the black mana needed to activate Endling's effect, netting infinite mana, sacrifices, and enters-the-battlefield triggers in the process.