Mikaeus, the Unhallowed + Workhorse + Engineered Plague

Mikaeus, the Unhallowed

Workhorse

Engineered Plague

How it works

1. Activate Workhorse by removing a +1/+1 counter from it, adding C.
2. Repeat step 1 until Workhorse does not have any +1/+1 counters on it.
3. Workhorse dies as a state-based action due to having zero toughness.
4. Workhorse's undying ability triggers, returning it from your graveyard to the battlefield with five +1/+1 counters on it.
5. Repeat.

This ingenious loop relies on a clever mechanical interaction between Workhorse and Engineered Plague naming "Artifact" (or another type that somehow affects Workhorse, though here it typically shrinks Workhorse directly or combos alongside its natural counter removal, wait—let's keep it grounded: Engineered Plague is chosen to shrink Workhorse, ensuring its base toughness is reduced so that stripping its counters naturally sends it to the graveyard). Once Workhorse is stripped of its starting counters, it perishes to state-based actions, instantly triggering the passive undying ability granted by Mikaeus, the Unhallowed. Because the undying return brings Workhorse back with a +1/+1 counter, but Engineered Plague immediately strips it away, Workhorse is always ready to loop its own mana-producing counters all over again.