"I know it." Rose's voice came forth. Her eyes were adjusting to the absence of light and now, thanks to the few photons that managed to penetrate through the cracks in the windows, she could see her classmates and the professor's silhouette. "They are Incubi, invasive beings that infest houses, attracted in particular to children. They never act during the day, but only appear at night, or in a very dark room. Muggles can see them too, but they usually mistake them for hallucinations."