Primal Prayers + Aetherstorm Roc + Cavern Harpy

Primal Prayers

Aetherstorm Roc

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 Aetherstorm Roc trigger.
3. Resolve the Aetherstorm Roc trigger, causing you to get E.
4. Resolve the Cavern Harpy trigger, returning it from the battlefield to your hand.
5. Repeat.