Mikaeus, the Unhallowed + Spawning Pit + Soulstinger

Mikaeus, the Unhallowed

Spawning Pit

Soulstinger

How it works

1. Activate Spawning Pit by sacrificing Soulstinger.
2. When Soulstinger dies, its undying ability and second ability trigger.
3. Resolve the second trigger, optionally putting -1/-1 counter(s) on another creature.
4. Resolve the Soulstinger trigger, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
5. When Soulstinger enters the battlefield, its first ability triggers, putting a -1/-1 counter on it.
6. Resolve the Spawning Pit ability
7. Repeat.