Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight + Sin, Unending Cataclysm + Umbral Collar Zealot

Prerequisites

You control another nontoken creature with base toughness 2 or greater that does not have a -1/-1 counter on it.

Steps

1. Activate Umbral Collar Zealot by sacrificing the additional creature.
2. When the creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Resolve the Umbral Collar Zealot ability.
4. Activate Umbral Collar Zealot by sacrificing Sin.
5. When Sin dies, it triggers twice.
6. Resolve the first trigger, returning Sin from your graveyard to the battlefield with a -1/-1 counter on it. As Sin enters, remove the -1/-1 counter from the creature sacrificed in step 1, causing Sin to enter with a +1/+1 counter on it instead.
7. Resolve the second Sin trigger, putting the counters it had on it when it was sacrificed onto a creature you control and failing to shuffle it into your library as it's no longer in your graveyard.
8. Resolve the Umbral Collar Zealot ability from step 4.
9. Repeat.