Infinite +1/+1 counters on a creatureInfinite +1/+1 counters on legendary creatures you controlInfinite creature ETBInfinite creature LTBInfinite creature sacrifice triggersInfinite death triggers
How it works
Activate Cauldron of Souls by tapping it, giving Derevi and Aerith persist until end of turn.
Activate Grimgrin by sacrificing Derevi.
When Derevi dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When Derevi enters the battlefield, it triggers, untapping Cauldron of Souls.
Activate Grimgrin by sacrificing Aerith.
When Aerith dies, it triggers twice.
Resolve Aerith's persist trigger, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Resolve the second Aerith trigger, putting a +1/+1 counter on each legendary creature you control, thus removing the -1/-1 counters from Aerith and Derevi.
Resolve the Grimgrin abilities from steps 2 and 5.