Nightblood - Elly Blake, Hodder Paperbacks

Ruby may be a Nightblood, the spawn of a deity hellbent on releasing his Minax from their prison. These beasts will roam the earth, devouring everyone until they are nothing but an empty husk. But Ruby is able to control the Minax to a degree and now she, Arcus, and Kai must find a way to bring Frostbloods and Firebloods together to make a stand against a foe more deadly than any they've faced