Mikaeus, the Unhallowed + Spawning Pit + Dread Tiller

Mikaeus, the Unhallowed

Spawning Pit

Dread Tiller

How it works

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