The Shadowbound King by Amy Pennza

Beware the shadows. Ever since I was a child, my father taught me to fear the elven kingdom that borders our lands. A place of twilight and falling leaves, the Autumn Court is rumored to be beautiful and deadly. No one knows what kind of creatures live in the thick, undying forests. No one knows what haunts the shadows.
But now those shadows have a name. And they’ve come for me.
Captured by the King of Autumn’s cruel bodyguard, Rane Laruthian, I’m carried into the heart of the shadow realm and dumped at the feet of King Andrin himself. As mysterious and dangerous as the shadows he commands, Andrin accuses my father of stealing the heart of Autumn’s power. Forbidden to enter the human realm, Andrin can’t make my father answer for his crimes—so he’ll take his revenge on me.
Surrender is unthinkable. I’m not without power, and I’ll do whatever it takes to prove my father’s innocence and win my freedom. But I didn’t realize what kind of revenge my captors had in mind. They’ll collar and keep me, molding me into the perfect weapon. They plan to use me against my father…but not until they use me for their own purposes first.

Caught in a web of magic and vengeance, I have to play a dark and seductive game. But when you dwell in the shadows, you discover that nothing is ever as it seems. As the game grows more deadly, I’m forced to question everything—including where I place my trust. And as I grow closer to my captors, I have to wonder if I really want to escape at all.