Dawn Breaker (Salvation Cycle book 2)

The king is dead. The world is lost. Long live the queen.
The thin membrane of magic separating the human and demonic planes has been destroyed. Nightrender, the immortal warrior of the gods, must find a way to rebuild it, but Hanne—the serpent girl, always too cunning to be trusted, too hungry for power—has become High Queen, and is too consumed with ambition to cooperate. Meanwhile, Rune—married to Hanne, but in love with Nightrender—is lost in the realm of demons after a disastrous battle, wandering alone in a  twisted landscape of mercury seas, black-glass spires, and winds blowing ash …
In this second and final installment of the Nightrender duology, the circle will close, and the world will be saved—or burnt to a cinder.