Mikaeus, the Unhallowed + Goblin Bombardment + Soulstinger

Mikaeus, the Unhallowed

Goblin Bombardment

Soulstinger

How it works

1. Activate Goblin Bombardment by sacrificing Soulstinger.
2. When Soulstinger dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Soulstinger enters the battlefield, it triggers, putting a -1/-1 counter on it.
4. Resolve the Goblin Bombardment ability
5. Repeat.

This engine exploits a clever counter-neutralizing interaction to turn Mikaeus, the Unhallowed and Goblin Bombardment into an endless loop of destruction. While Mikaeus grants your non-Human creatures undying so they return with a +1/+1 counter, Soulstinger's entry trigger places two -1/-1 counters on itself. State-based actions immediately annihilate both opposing counters, resetting Soulstinger's state so it can safely die to Goblin Bombardment again and again.