Transformation can play a big role in frightening stories because transformation in and of itself can be frightening. This process involves the unknown and a loss of control, two things that very much tend to frighten human beings who prefer the familiar and secure.

Body horror is one example of a terrifying transformation where the physical body turns against the individual, becoming something grotesque and alien. Modern horror movies such as The Human Centipede or The Fly showcase this kind of transformative horror, but its origins go back at least as far as Frankenstein, where Victor transforms a collection of rotting body parts into a living creature.

Transformation can also be internal. The Strange Case of Dr, Jekyll and Mr. Hyde derives its horror less from the crimes of Mr. Hyde and more from the idea that a normal person like Dr. Jekyll could be capable of such barbarity deep down. The transformation, rather than affecting the body, affects the mind and soul, turning an ordinary man (maybe not so different from the reader) into a beast.