I experienced this "sleep paralysis" one time when I was young.
I had been asleep for probably about an hour, when I heard my father, splashing about in the sink in the bathroom across the hall from my bedroom, doing his getting ready for sleep routine. The door to my room was open letting the light from the hallway stream into my room.
I looked at the light streaming in onto the floor of the shag carpet of my room from my position in my bed, lying on my side with the covers pulled up under my chin. Then I looked deeper into the room - onto the other side away from the light of the door - to the floor just beneath a window seat there. As my eyes became focused there, I noticed a gathering pool of darkness there - a black void. Out of the void in the floor a figure rose up out of the floor. It appeared to be robed and hooded with a wraith-like shimmer about it. I could only see darkness where the face would be as it appeared to glide across the floor towards the doorway where it stopped. It seemed to be looking across the hallway into the bathroom at my father, who, I could tell from the clarity of the sounds coming from the bathroom, hadn't shut the door.
I was feeling very frightened at this point and I remember slowly trying to shift myself under my covers and I was able to move at this point. The specter turned towards me and I had my eyes squinted closed, but I could still see through the tiny gap I left through my eyelashes. At this point I became paralyzed and I couldn't have moved if I wanted to. It glided over the floor to just in front of my face at the edge of my bed and stopped gazing down at me for a few moments, then it turned and went across the room to where my brother was sleeping and looked down at him for a moment or two and then turning back towards the window it glided towards the window sliding down into the floor as it went that way - disappearing. At the moment it left the room I could once again move and I jumped up and went into the bathroom to ask my father if he had come into my room - even though I knew he hadn't.