Mairsil, the Pretender + Blood Vassal + Endling + Barrenton Medic

Mairsil, the Pretender

Blood Vassal

Endling

Barrenton Medic

How it works

1. Activate Mairsil using Endling's third ability by paying , giving Mairsil undying until end of turn.
2. Activate Mairsil using Blood Vassal's ability by sacrificing it, adding .
3. Mairsil dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
4. Activate Mairsil using Barrenton Medic's second ability by putting a -1/-1 counter on it, untapping it.
5. Repeat.

This ingenious loop turns Mairsil, the Pretender into a self-sustaining engine by utilizing the activated abilities of exiled cards. By caging Blood Vassal, Mairsil gains the ability to sacrifice itself for black mana. Because Mairsil has also absorbed Endling's undying ability, its demise instantly triggers a return to the battlefield equipped with a +1/+1 counter. To clear that counter and keep the cycle going, Mairsil taps into Barrenton Medic's ability to place a -1/-1 counter on itself, which immediately neutralizes the +1/+1 counter while untapping Mairsil to repeat the entire process for infinite black mana.