Elias slammed his fists against the glass, but the only sound was the clicking of the apartment door locking from the outside.
He stepped into the hallway, his heart hammering against his ribs like a trapped bird. The sheet lay in a heap. The mirror was bare. In the dim moonlight, he saw himself—or rather, the thing that looked like him—standing inside the glass. Horror Story
Start with small, "wrong" details and gradually increase the tension until the final reveal or twist. Elias slammed his fists against the glass, but