Thurl's main motivation and goal was power. He thought that, thanks to Yan-C-Bin, he could rule Waterdeep. Thurl pledged loyalty to Aerisi Kalinoth but only reluctantly, believing that one day he would use her and her minions to conquer Waterdeep.
Thurl was once a griffon rider in Waterdeep until a great storm nearly killed him. After that experience, he started to worship Yan-C-Bin. Soon, he gathered around him a group of like-minded wealthy folk of Waterdeep and founded the Feathergale Knights as a cover group of the Cult of the Howling Hatred.