What is a transformation?
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.