Primal Prayers + Decoction Module + Cavern Harpy

Primal Prayers

Decoction Module

Cavern Harpy

How it works

1. Cast Cavern Harpy by paying E due to Primal Prayers, if able, or by paying its mana cost.
2. When Cavern Harpy enters the battlefield, it and Decoction Module trigger.
3. Resolve the Decoction Module trigger, causing you to get E.
4. Resolve the Cavern Harpy trigger, returning it from the battlefield to your hand.
5. Repeat.