Mikaeus, the Unhallowed + Viscera Seer + Heartmender

Mikaeus, the Unhallowed

Viscera Seer

Heartmender

How it works

1. Activate Viscera Seer 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 Viscera Seer ability.
4. Activate Viscera Seer 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 Viscera Seer ability.
7. Repeat.