Mallory is a mentally ill ex-Catholic teacher who has spent the last few years working a job he's overqualified for, not taking his pills, and living with his demonic roommate, with whom he is absolutely not in love.
Pyre is a demonically-possessed demon, born into an underworld known as the Wretched World and possessed at birth by the murdered cohort of the worst dictator the Wretched World has ever known. He escaped into the Human World to try and contain the monster, known as the Fire, inside him.
When the Fire gains more control over Pyre than he's ever had, Pyre realizes he has to leave the Human World and sacrifice himself in the Wretched World, and Mallory determines that if it's a choice between Pyre and humanity, he'll take Pyre and set the rest on fire if he has to.
In the first book in this 4-part series, Mallory and Pyre fight with Mallory's family and Pyre's past as they slowly delve beneath the surface of the lies they've been telling each other.