Mara from European Superstition

Nightmare or Mare - waking with shortness of breath or a heaviness in the chest is evidence of a visit by a nightmare demon. These heinous creatures attack at night, sitting on a person's chest and breathing putrid breath into their nostrils. Their exhaust causes horrible dreams; and if the person awakes while the demon is still present, he or she will be paralyzed under the weight of the monster.