Bright Blinding Ruin by Candace Wondrak

A girl who doesn’t want to save the world.
Rey never wanted to be a hero, but she can only refuse the call so much before it drags her into the fray, kicking and screaming. She’s the only hope for Laconia, and without her, its remaining people will fall—including Frederick, the man who puts more faith in her than he should.
She’s not a hero. She’s just a college student in way over her head.

An indescribable evil so powerful no past empress could defeat him.
Invictis is the undefeated, the inevitable. He is not a person but a weapon whose only purpose is to destroy Laconia. Now that he is whole once again, he can finish what he began twenty years ago.
Only things are different. Rey is different, and her journey will take her back through the realms of Laconia as she searches for the truth.

A predetermined bond neither of them can break.
Invictis haunts her dreams, her memories, turning his wrathful ire upon her instead of Laconia with an almost obsessive need to see her fall. As he hunts her across the land, she discovers there’s so much more to the story than she knew. This fight is as old as Laconia itself.