Atonement of the Spine Cleaver - F E Bryce

Rorax Greywood lives in the Realms and was trained to fight, kill, and steal for the House of Ice's special forces unit, the Heilstorms, from the time she was ten years old. During her time as a Heilstorm, Rorax committed more violent atrocities in the name of her House than she could ever remember, but the blackest stain on her soul remains the Siege of Surmalinn-an infiltration and near genocide in the largest House of Death city.
Years later, when Rorax is called upon by the Gods to compete in the Choosing, a deadly competition that decides the next Guardian of the Realms, it quickly becomes clear the only House strong enough to protect her is the House of Death.
Rorax knows she isn't worthy to become the Realms' new Guardian, so she somehow needs to convince the House of Death-and their brutally handsome lieutenant-to both help her find the next suitable Guardian, and free her from the Choosing before the Choosing kills her.