A man in Al Mamoon who goes by the name of Derwin asked you to bring him some creatures from the desert.
This errand is available to complete immediately after completing the Temple of Trials.
To begin this errand, you need to find Derwin the academic researcher on the east side of Al Mamoon. He's a researcher of familiars and needs you to catch some. To be more precise, he requires a Green Buncher and a Turbandit.
For your reference, the Green Buncher is the large, green, vegetable-like familiar while the Turbandit is the small, purple fellow with, well, a turban. Both can be found in the desert, and you'll most likely have to fight a lot of them before they can be caught by Esther's Serenade. When you've finally succeeded in capturing both familiars, return to Derwin to claim your reward.