Mikaeus, the Unhallowed + Umbral Collar Zealot + Heartmender

Mikaeus, the Unhallowed

Umbral Collar Zealot

Heartmender

How it works

1. Activate Umbral Collar Zealot by sacrificing Heartmender.
2. When Heartmender dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Resolve the Umbral Collar Zealot ability.
4. Activate Umbral Collar Zealot by sacrificing Heartmender.
5. When Heartmender dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. Resolve the Umbral Collar Zealot ability.
7. Repeat.