The transformation in a story or really anything in a scary situation. Like when the main character is in his/her room chilling and he/she goes downstairs to get a drink when someone bursts into the house and causes him/her to run out of the house or into a room with a lock. When something calm in a story jumps into a scary moment of fear, and then just goes back to normal.
The first book about transformations is the well-known work "Metamorphosis" by Ovidio. The transformation implies a change. In literature, you can find a transformation when an object passes from a state A to a state B. For example, in a horror story a person can go from being the friend of the protagonist to being possessed.
The transformation in the horror stories is unexpected. The surprise factor is essential in this cases. The horror stories present the circumstances in such way so as later can be different.