Well, shadow play has been around for a long time and has a certain old - world charm that can be used to great effect in a scary story. It can be associated with ancient, forgotten tales. The way the shadows can be manipulated to appear suddenly or disappear just as quickly can give the viewer or listener a real fright. It's like something is there one moment and gone the next, leaving you with a feeling of dread and anticipation of what might come next.
Shadow play can create an atmosphere of mystery. In a dark room with only the light for the shadow play, it's easy to feel a sense of unease. The fact that the figures are two - dimensional but seem to have a life of their own adds to the spookiness. Also, the lack of color makes everything seem more somber and menacing. You can't really tell what the shadow might be made of or what it's intentions are, which is quite frightening.
The ambiguity of shadows. Since they are just silhouettes, our minds can fill in the blanks with the scariest things possible. For example, a simple shadow of a person can be made to look like a monster with just a little distortion in the movement or shape.