Mikaeus, the Unhallowed + Barrenton Medic

Mikaeus, the Unhallowed

Barrenton Medic

How it works

1. Activate Barrenton Medic's second ability by putting a -1/-1 counter on it, untapping it.
2. Repeat step 1 until Barrenton Medic dies as a state-based action due to having zero toughness.
3. Barrenton Medic dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
4. Repeat.