Derevi, Empyrial Tactician + Cauldron of Souls + Aerith Gainsborough + Spawning Pit

Derevi, Empyrial Tactician

Cauldron of Souls

Aerith Gainsborough

Spawning Pit

How it works

1. Activate Cauldron of Souls by tapping it, giving Derevi and Aerith persist until end of turn.
2. Activate Spawning Pit by sacrificing Derevi.
3. When Derevi dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
4. When Derevi enters the battlefield, it triggers, untapping Cauldron of Souls.
5. Activate Spawning Pit by sacrificing Aerith.
6. When Aerith dies, it triggers twice.
7. Resolve Aerith's persist trigger, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
8. 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.
9. Resolve the Spawning Pit abilities from steps 2 and 5.
10. Repeat.