Mikaeus, the Unhallowed + Spawning Pit + Reluctant Dounguard

Mikaeus, the Unhallowed

Spawning Pit

Reluctant Dounguard

How it works

1. Activate Spawning Pit by sacrificing Reluctant Dounguard.
2. When Reluctant Dounguard 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.