Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight + Sin, Unending Cataclysm + Altar of Dementia

Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight

Sin, Unending Cataclysm

Altar of Dementia

How it works

1. Activate Altar of Dementia 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 Altar of Dementia ability.
4. Activate Altar of Dementia 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 Altar of Dementia ability from step 4.
9. Repeat.