Mikaeus, the Unhallowed + Spawning Pit + Gnarlbark Elm

Mikaeus, the Unhallowed

Spawning Pit

Gnarlbark Elm

How it works

1. Activate Spawning Pit by sacrificing Gnarlbark Elm.
2. When Gnarlbark Elm 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.