The Mora is a malicious entity of Germanic and Slavic folklore that sits, walks, or "rides" on the chests of sleeping people, bringing on nightmares. Placed upon the chest, it is said to have a suffocating effect and can cause cold sweats and sleep paralysis.
It may be the soul of a person, living or dead. The Polish mora and Czech můra denote both this nightmare spirit and a moth, and the word lies at the root of the English "nightmare."