Mikaeus, the Unhallowed + Spawning Pit + Heirloom Auntie

Mikaeus, the Unhallowed

Spawning Pit

Heirloom Auntie

How it works

1. Activate Spawning Pit by sacrificing Heirloom Auntie.
2. When Heirloom Auntie 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.