Mara

The mara (Old Norse, from which 'nightmare' derives) is a spirit — often the projected soul of a living person — that rides sleepers at night. In Scandinavian, Germanic, and Slavic traditions she is a woman who transforms after dark and travels to press down on the chests of those she desires or hates. She could ride horses until they were exhausted, braid their manes into tangles ('mara-plaits'), and even afflict cattle.

Often perceived only as a crushing weight on the chest — invisible. When visible, she appears as a pale woman, sometimes with wild hair, crouching on the sleeper's chest. She enters through keyholes and cracks under doors.

Caused sleep paralysis, terrifying dreams, and suffocation. Could ride a person for multiple nights, causing wasting illness. Could affect horses and cattle. In some traditions she was a living woman whose spirit left its body — if her name was kept in your hand as she entered, she would be trapped.