I was twelve years old, I shared one room with my sister, and at the end of that room was one closet. We kept our beds as far away from that one closet as possible, placing them close together so we could reach each other easily, because the closet had a tendency to open at night, just as were falling asleep, and let something in.
We kept it closed, barricaded it, and even had our mother check it a few times before the lights went out, but it still opened and let it in.
"It" was dark. It was all we could call it, for there was nothing else to see but a deep black that would come out of that closet and creep across the room toward us. Some might call it a black mist, but it didn't spread out across the floor, or swirl in the air as it came, just a moving mass of a deeper black than the underside of our beds.
There was no chill in the air when it came, but a chill in the bones. You felt terrible, sick to the stomach, and all that pain and terror was eminating from the dark. But even though you wanted it to go back to the closet, you had to wait until it was completely in the room, almost onto the bed before you could yell at it to go back. Then it would slip back in and evaporate into the shadows. The door would remain open all night for neither I nor my sister would dare go shut it.