Mikaeus, the Unhallowed + Spawning Pit + Plague Belcher

Mikaeus, the Unhallowed

Spawning Pit

Plague Belcher

How it works

1. Activate Spawning Pit by sacrificing Plague Belcher.
2. When Plague Belcher dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Plague Belcher enters the battlefield, it triggers, putting a -1/-1 counter on it.
4. Resolve the Spawning Pit ability
5. Repeat.