Derevi, Empyrial Tactician + Cauldron of Souls + Aerith Gainsborough + Umbral Collar Zealot

Derevi, Empyrial Tactician

Cauldron of Souls

Aerith Gainsborough

Umbral Collar Zealot

How it works

1. Activate Cauldron of Souls by tapping it, giving Derevi and Aerith persist until end of turn.
2. Activate Umbral Collar Zealot 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 Umbral Collar Zealot 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 Umbral Collar Zealot abilities from steps 2 and 5.
10. Repeat.

This loop relies on carefully managing counters and sacrifice mechanics to continually recycle your key threats. Cauldron of Souls grants persist to both Derevi, Empyrial Tactician and Aerith Gainsborough, allowing them to return from the grave when sacrificed to Umbral Collar Zealot. As Derevi enters the battlefield, its triggered ability untaps the Cauldron so you can keep the engine running, while Aerith's death trigger distributes +1/+1 counters that neatly cancel out the -1/-1 counters from persist.