The Daughter of the Scarred Elf by Maham Fatemi

The half-elf’s army will rise, and when it does, all will bow down to the fae queen or perish.

When Kolfinna is put on trial by the king for her questionable loyalties, her only savior turns out to be Sijur Bernsten, the enigmatic lieutenant general who offers her a way out. The price? She’s bound to him for fifteen years with an ancient fae rune.
But Kolfinna soon finds that Sijur has his own sinister plans and ambitions of power, and she’s his means to reach it. With more and more innocent blood staining her hands, she must find a way to break the magic binding her to him, or continue down a road of darkness.
As Kolfinna tries to balance her new position in the military, she has to come to terms with her connection to the cruel half-elf commander, and what it means for her closest relationships–her budding romance with Blár Vilulf, her new friends in the military, and her place in a world that hates the fae.