It’s like when people say Chinese people eat dogs and cats and Mexican people eat tacos
In the words of Geoffrey Ase "One reason for their becoming popular was that they appealed to a wide variety of interests, in an age when there wasn't much in the way of imaginative fiction" The arthurian story have magic, fantasy, religion, drama, comedy and different topics that are appealing for almost everyone. The success of this epic story is its timing release, the wide topics that covers and that it is an easy reading for people of all ages.
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.