Mikaeus, the Unhallowed + Spawning Pit + Brambleback Brute

Mikaeus, the Unhallowed

Spawning Pit

Brambleback Brute

How it works

1. Activate Spawning Pit by sacrificing Brambleback Brute.
2. When Brambleback Brute dies, its undying ability triggers, returning it from your graveyard to the battlefield without a +1/+1 counter on it due to its replacement effect.
3. Resolve the Spawning Pit ability
4. Repeat.