Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight | Sin, Unending Cataclysm

Cards

All permanents on the battlefield (
Eirdu as Isilu, Carrier of Twilight and
Sin and
Thermopod without a -1/-1 counter on it).

Activate
Thermopod by sacrificing it.
When
Thermopod dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Activate
Thermopod by sacrificing
Sin.
When
Sin dies, it triggers twice.
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
Thermopod, causing
Sin to enter with a +1/+1 counter on it instead.
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.
Repeat.

This combo has been estimated to be Exhibition (Bracket 1+).