The Old Hag Syndrome: known throughout the world and in many cultures and can vary in degrees from a somewhat mild case, like I experienced, to a much more extreme episode where a sleeper wakes up paralyzed and sees a spectral presence moving over them. The victim can feel pressure on their chest and possibly even experience choking.
I myself have experienced, not an old hag, but what I remember calling myself years back "an entity" as well, that was also dreamed of by my then wife. In my dream, this black mass appeared at the foot of my bed, and then with unimaginable speed lept upon me and I seized it with both hands just before it would reach me. I then fought with this and threw it with everything I had out of the room where it then dissapeared. It was such a strange and vivid dream that I mentioned it to my wife, who then told me she had the same dream and the entity was choking her. I have not experienced anything like that since.
The film "The Entity" above shows were science and the paranormal collide, and the struggle to explain why people all over the world are experiencing the same type of night terrors. Scientists believe this phenomenon is simply "sleep paralysis", while some believe that old hag syndrome is an actual spectral attack by a demon.