The Oubliette written by J C Stearns.
With the death of Ruprekt Matkosen, his daughter, Ashielle, is now the Lord Governor of Ceocan. Her father’s murderers still lurk in the shadows, threatening not only her rule but every mortal soul under her protection. Even her own people cannot be trusted – any one of them may be part of the poisonous plot to destroy her bloodline. Deep beneath the palace, locked away from all human contact, Ashielle finds a weapon unlike any other: a monster, more adept at hunting in the darkness than any assassin. Allying with such a horror is surely blasphemy, but with doom skulking around every corner, Ashielle is forced to revive an ancient pact with the beast. Yet she soon discovers that her family’s mortal enemies are not the only evil that hungers to consume her.