Anthony Horowitz - Raven's Gate

Young Adult - Power of Five, Book 01
Matt has always known he has unusual powers. Raised in foster care, he is sent to Yorkshire on a rehabilitation programme, only to find himself in the midst of sinister goings-on. Matt investigates and uncovers a terrible secret - eight guardians are protecting the world from the evil ones, beings banished long ago by five children. But devil worshippers want to let the evil ones back in. As the story reaches its climax, it looks like Matt has succeeded in stopping them ... or has he?