Nightblood

Ruby's world has changed more than she ever could have imagined. She's in love with a powerful Frost King. She's the heir to the Fire Throne. And she may be a Nightblood—the spawn of a vengeful deity hellbent on releasing his wraithlike Minax from their prison. Once freed, these beasts will roam the earth, devouring every last person until he or she is nothing but an empty husk. But Ruby is able to control the Minax to a degree, and now she, her beloved Arcus, and her friend Kai must find a way to bring Frostbloods and Firebloods—sworn enemies—together to make a stand against a foe more deadly than any they've faced.