In Germanic folklore, a mara is an evil spirit that sits on your chest while you sleep, constricting your air supply and turning your dreams into nightmares. It appears in some form or another in all the Germanic cultures, although the specific name and description change with each language.
Most notably, the English word for the mara is “mare,” which is theorigin of the word nightmare. Croatians believe that the mara takes the form of a beautiful woman at night. She visits men in their sleep to torture them and slowly suck away their life force over the course of a decades. In other cultures, a mara is a thick-skinned goblin.