Curse of Shadows and Thorns by LJ Andrews

She knew her people stole the crown from the fae. She had no idea they’d use her to steal it back.

As the king’s niece, Elise Lysander must secure a powerful marriage match to strengthen her uncle’s influence. If she refuses, her deathly-ill father will pay the price.
Trapped by her sense of duty, Elise reluctantly puts her fate into the hands of Legion Grey, the handsome and mysterious dowry negotiator. But soon Legion incites a blistering, forbidden passion she can’t ignore. As their attraction grows, so do the dangers: strange attacks by black-eyed people and whispers of lost fae magic.
When a bloody coup upends the kingdom, Elise flees with Legion, but she soon discovers the man she let into her heart has his own magical secrets and plans. And he has every intention to use Elise to see them through.
With war between magic and mortal on the horizon, Elise must decide her place. Does she fight against magic once more? Or do her loyalties lie with a man whose past might save them…or bring her destruction?