The Darkness Within Us by Tricia Levenseller

Chrysantha Stathos has won.

By hiding her intelligence and ambition behind the mask of a beautiful air-headed girl, she has gathered important secrets, earned her father’s constant care, and become a wealthy duchess—all by age nineteen. Now, she is only one elderly, dying duke away from having all the freedom, money, and safety she’s ever wanted.

Or so she thought.

Turns out her little sister somehow snatched the true victory away from her: Alessandra is wedding the Shadow King, the most powerful man in the world. Meanwhile, after the death of Chrysantha’s no-good, lecherous husband, a man claiming to be the duke’s estranged grandson turns up to inherit everything that should be hers.

Chrysantha is enraged. There is no way that she’s going to let some man take everything from her. Never mind that he’s extremely handsome and secretive and alluring with mysterious powers… No, Chrysantha wants Eryx Demos dead, and in the end, a Stathos girl always gets what she wants.