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.
In Newfoundland, sleep paralysis is known as the 'Old Hag'. The Hag can be summoned to attack a third party, like a curse. A way to call the Hag is to recite the Lord's Prayer backward according to folklore.
Sleep paralysis is a phenomenon in which an individual, either during falling asleep or awakening, temporarily experiences an inability to move, speak, or react.