House of Gilded Nightmares by Aurora Grey

A Shadow-Fae guardian, the future Queen of Nightmares, and the dark destiny they share…

Sarielle has lived behind the walls of the Amethyst Palace her whole life, training to one day become High Priestess of Eldare. But on the night of her initiation, a dark stranger—the man who’s appeared in her dreams for years—arrives to spirit her away to another realm.
Zyren, a shadow-fae lord, is her blood-sworn guardian. His job is not only to protect her, but to take her to the King of Nightmares, her betrothed, to claim her birthright as queen. And she’s not about to go quietly.
As Sarielle and Zyren traverse the treacherous Nightmare Realm, they’re hunted by terrifying monsters and enemy courts who want Sarielle as much as the king does. To escape her enemies, Sarielle will have to claim the dark power she was born with, a magic which is quickly unraveling the careful control she’s maintained her whole life. As is Zyren, who awakens a fiery longing for something she knows she can never have.

Because if she doesn’t claim her throne, she dooms not only her realm, but their entire world.