Night hag

A night hag is a fantasy creature from the folklore of various peoples which is used to explain the phenomenon of sleep paralysis. It is a phenomenon during which a person feels a presence of a supernatural malevolent being which immobilizes the person as if sitting on his/her chest or the foot of his/hers bed. The word "night-mare" or "nightmare" was used to describe this phenomenon before the word acquired its modern, more general meaning. Various cultures have various names for this phenomenon and/or supernatural character.