Thief's Heartache (The Imperial Series) - Imperial, #3

As the eldest daughter of the Crown Prince Damuk of the Abital Empire, Marella has never been out from under her father's penetrating gaze. But after months of begging, her father has finally allowed her to go west to the Yarian Republic. Now at the court of her uncle, King Darian, Marella finds herself as bored as she ever was at home. For now. William, twin brother of the powerful Lord of Ravenscar, has spent his entire life following his brother.
But his brother's brutality is getting worse and William is losing faith and regretting his promises of long ago. As his brother begins down a path of cruelty and depravity, William realizes there are places he cannot go. Thrown together by fate and a madman's lust for power, William and Marella must join forces. But even together they can do no more than try desperately to keep their distance. With no resources and no allies, can they weather the storm?