Now here comes the creepy thing: not to sound like a hipster but I once had a pre-sleep hallucination. Guess what was in it.
What is weirder about this is that I later looked for info about this "event" and found the creepypasta about slendy, which pretty much describes in detail what I saw, except for the white face, mine was a faceless pile of mist which formed out of the shadows, with a suit and tentacles. Cool, uh?
Note: I was completely sober AND that never happened to me again, just that time.
I guess the explanation for it may be that our brains are unimaginative and we are likely to dream scary things like that (for example the images of "have you seen that man" which depicts a rather ugly individual which people tend to dream with)
Man: