Mikaeus, the Unhallowed + Spawning Pit + Reaping Willow

Mikaeus, the Unhallowed

Spawning Pit

Reaping Willow

How it works

1. Activate Spawning Pit by sacrificing Reaping Willow.
2. When Reaping Willow 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.