Chronicles of M - Book 2 - Ammit
Thomas and Samuel race to find a way to expel the demon that controls M and fail. It escapes, disappearing in broad daylight. With Thomas unable to track it down, magically or with science, he's forced to do the one thing he hoped to not do. Thomas has to rely on the advice of a all-powerful being that he wants nothing to do with and use it's 1400 year old demon slayer to hunt down the demon in M.