The Wraith King (The Rise of Northgall, #1) by Juliette Cross

A brutal, bloody war against the ruthless Wraith King has cost the light fae more than innocent lives. Una Hartstone, Princess of Issos, learns the price the Wraith King demands to end the war once and for all. Her. In exchange for the safety of her people, she agrees to give her life-and her body-to her greatest enemy.

Gollaya Verbane is determined to fulfill his destiny and his god's prophecy. When his seer points to the Princess of Issos as the key to the rise of the dark fae, he demands her submission. But when she finally yields, he realizes Una is much more to him than a priceless weapon.

A mystery that has haunted Una for years awakens when she is abducted and dragged back to Näkt Mir. The palace hides many dark secrets...and at least one traitor. A traitor determined to take King Goll's throne-and all he possesses. What he doesn't know is that Una's magick is more powerful than he can imagine, and that Goll will burn the whole world to save her.