Mikaeus, the Unhallowed + Spawning Pit + Pith Driller

Mikaeus, the Unhallowed

Spawning Pit

Pith Driller

How it works

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