Mairsil, the Pretender + Endling + Cauldron of Souls + Blood Vassal + Urabrask the Hidden

Mairsil, the Pretender

Endling

Cauldron of Souls

Blood Vassal

Urabrask the Hidden

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.