We had a large, empty bedroom that my parents were always going on about turning into various things (a guest room, a game room, a gym, etc) but never did anything with. It used to be my room, before they kicked me out into the closet-sized third bedroom in order to never do anything with my old room, but that's by the by. It was always a creepy room, but it was even creepier now that it was empty.
Unfortunately it was right next to my new room, and I had to walk past it every time I wanted to use the bathroom or go downstairs. It had a really nasty atmosphere to it, and my mother insisted on keeping the door open so that the air could flow freely around the house or whatever. I'd try to avoid looking at it.
I was home alone a lot when I was younger, and this time my parents were both away for around a week or so. Over the course of this time that bedroom had been getting steadily creepier, and one evening I walked past it and I couldn't help but look. I don't know why, I just looked before I could think about it. It was really dark in that room, because for some reason the curtains were always closed, and for a moment I couldn't see anything. Then, as my eyes adjusted, I slowly made out a kind of... mass, in the top left corner of the room.
It was huge, filling up the whole corner, and it looked like it was constantly moving. Imagine if someone got a pen and managed to scribble in the air; just a huge mess of black lines all tangled together. It looked like that, just hovering there. I stared at it for a moment, and then I felt the most unbelievable sense of dread, and I reached out and closed the door. I didn't open it again until before my parents got back.
I didn't mention it to them, because I didn't think they'd believe me. The bedroom was eventually turned into a guest room, and wouldn't you know it? Everyone who slept in there had some kind of horrible experience they couldn't explain. Some people reported feeling uneasy or watched all night; other people reported seeing an old man sitting on a chair in the corner of the room (there was no chair there). My mother, on several occasions when sleeping in there so she didn't disturb my father getting up early for work, saw the old man in the chair, and also saw him standing inside the mirror banging his fists against the glass. I myself had to stay in there once, and woke up with red scratch marks down the length of my leg.