The Room Upstairs — Iain Rob Wright

You can't choose your family, but sometimes they're all that stand between you and utter darkness. Martin Gable is a boy becoming a man, but nothing can prepare him for the evil that has just entered his home. A mysterious doorway has appeared on the landing outside his bedroom. Something is alive inside of it. Something old and extremely hungry. As old resentments arise and home becomes a dangerous place, Martin must become a man and face down the monsters his family has brought home with them. Before it's too late. Supernatural chiller novel from master of the genre, Iain Rob Wright.