Rose in Chains by Julie Soto

The war is over, the dark forces have won, and the hero who was supposed to save them is dead. Captured as her castle is overrun by the enemy, Princess Briony Rosewood is stripped of her magic and auctioned off to the highest bidder. She's sold to Toven Hearst: scion of a family known for their cruel control of magic - and her long-time, and unresolved, infatuation.