Shadow Realm by Quinn Blackbird

Cliff has changed from captor to lover, then to protector.
He’s gone against his own people, shed dark fae blood–to save me.
But now who will save him?
With these crimes, we run from his unit and into the black world. We have one mission, one drive left to push us; reach his world and ally before we are both captured.
If we are … no one can save either of us.