Complex sentence: Cole goes to investigate a mirror in the room, when suddenly the mirror smashes open and the real adult Brahms crawls through a shattered mirror-he was living within the walls of the house the entire time.
Split into simpler sentences: Cole goes to investigate a mirror in the room, when suddenly the mirror smashes open and the real adult Brahms crawls through the shattered mirror. He was living within the walls of the house the entire time.