Beauty in Darkness by Elizabeth Briggs

They’ve been enemies their entire lives, their two kingdoms locked in a never-ending war—until an arranged marriage sparks a passion they can’t ignore…
To save her father’s life, Princess Rose must wed the handsome, brooding wizard king Raith, her kingdom’s greatest enemy. Though she always knew she’d be married off in a political alliance, she never expected it to be to that monster. Or that the only time she would see her elusive husband is when he’s teaching her magic, treating her more like his student than his wife.
King Raith may have taken a bride to end a war and protect his people, but after his former wife’s murder he has no interest in falling in love again, especially when Rose defies him at every moment. But Rose’s beauty and boldness tempt him to open his heart a second time, and every day it gets harder to resist her.
Just as Rose begins to see another side to Raith, she discovers her new home holds deep, dark secrets that threaten the entire land, secrets no one dares speak of—and Raith is at the center of them. As their desire for each other grows, Rose will do whatever it takes to uncover the truth about his past and the shadows spreading across the land. But when darkness threatens to tear them apart, they’ll have to find a way to stop it together—or doom their kingdom forever.