Mirrors.
In Western superstition, it was believed that Mirrors held evil spirits. In Chinese superstition, it was believed that spirits could not appear in mirrors, as they didn't like being seen. A more modern belief is that mirrors are actually showing parallel worlds.
Quantum Physics says that the Universes are Infinite. Everything you do creates a separate universe, even something as minuscule as a cell splitting one way. Every choice creates a branching path. Every choice has an impact. The Modern belief is not wrong. As for the superstitious ones? The Western belief is correct. Your universe might not have spirits, but the alternate one does. If you're awake at night, and you hear voices, that's an alternate universe screaming for help. Trying to break into yours.
The only way to stop it is to sever the ties, forever. When you kill one form of you, both die.
The girl was staying in the mansion. She had been cataloging her inheritance from her great aunt. She was the only heir. She slept in a back bedroom with an antique mirror. The room's wallpaper was stained with what she had thought was mold.
It was actually blood.
Anout 60 years before, a murder had occurred in that back room. The daughters of the family were murdered by a vagrant. The suspect was never caught. The only thing in the room was a mirror, which caught the souls, trapped them in the parallel universe. The room then went unused.
The girl had chosen the room for its view. She could wake up the next morning, see the ocean, and then get to work cataloging. She also chose it for its point in the house. The oldest wing in the mansion.
The first night, the stay wasn't so bad. She just heard some voices. She thought it was the house settling, as old houses do. The second night, she slept with a knife by her side. She didn't know why. It was just an uneasy feeling she had. She slept again. The third night, she had learned the story of the room, but still decided to sleep in it. She liked the room, and she liked its history. An old house was bound to have something like that.
That night, she did not sleep peacefully. The voices sounded vengeful. She clutched the knife close to her, and listened as the whispers surrounded her. They sounded angry, and murderous, and they called her name. She finally stood up. She slowly walked to the mirror.
The girl stabbed the knife into the mirror. She didn't know why she didn't just smash the mirror. She stabbed it. It began to fracture. Blood came dripping out of the mirror. She dug the knife deeper. She began feeling a burning sensation as she dragged it across her reflection. She began bleeding herself. She screamed in pain, and collapsed. The mirror fell as well. It shattered into pieces.
The next day, Several police Detectives entered the old house. It smelled musty, and old. Books lay scattered all over the Living Room, cobwebs and dust covered everything, Some things looked like they haven't been touched in years. They made their way to the back room of the house. The detectives looked at the body.
"Damn shame that she died so young." One of them said.
The other nodded. "Let's get CSU in here, and leave. I don't like this place at all."
"Right."
And with that, the detectives stepped out from the room.