Szarel, Genesis Shepherd + Cauldron of Souls + Clock of Omens + Umbral Collar Zealot

Szarel, Genesis Shepherd

Cauldron of Souls

Clock of Omens

Umbral Collar Zealot

How it works

1. Activate Cauldron of Souls by tapping it, giving two nontoken artifact creatures persist until end of turn.
2. Activate Clock of Omens by tapping the nontoken artifact creatures, untapping Cauldron of Souls.
3. Activate Umbral Collar Zealot by sacrificing one of the nontoken artifact creatures that doesn't have a -1/-1 counter on it.
4. When the creature dies, its persist ability and Szarel triggers.
5. Resolve the persist trigger, returning the creature from your graveyard to the battlefield.
6. Resolve the Szarel trigger, putting a number of +1/+1 counters on the other artifact creature.
7. Resolve the Umbral Collar Zealot ability.
8. Repeat from step 3 once, instead sacrificing the creature who had counters placed on it in step 6.
9. Repeat from step 1.

This intricate loop centers around Cauldron of Souls granting persist to your artifact creatures, allowing them to cheat death and return to the battlefield with a -1/-1 counter. By sacrificing those creatures to Umbral Collar Zealot, you trigger Szarel, Genesis Shepherd, whose ability distributes +1/+1 counters that instantly neutralize the persist -1/-1 counters upon the creatures' return. Meanwhile, Clock of Omens harnesses the tapping of these looping artifact creatures to constantly untap Cauldron of Souls, keeping the engine running indefinitely.