The Witch King
(Book)

Series information from Goodreads.

Wyatt, a transgender witch, hides in the human world after he loses control of his magic, but his fiancé, Emyr, a fae prince, is at risk of losing his throne if he does not find and marry Wyatt.

In Asalin, fae rule and witches don't. Wyatt's betrothal to his best friend, fae prince Emyr North, was supposed to change that. But after Wyatt lost control of his magic, he fled to the human world. Now a coldly distant Emyr has hunted him down. Despite transgender Wyatt's newfound identity and troubling past, Emyr has no intention of dissolving their engagement. In fact, he claims they must marry now or risk losing the throne. As Wyatt realizes the boy he once loved may still exist, he must decide once and for all what's more important: his people or his freedom. -- adapted from jacket