We can't say for sure that 'A Demon Within Based on a True Story' is a horror story. While the presence of a 'demon within' sounds like it could be part of a horror narrative, it could also be a drama or a mystery that just uses that phrase to describe an internal conflict within a character based on real - life events. For example, it could be about a person's struggle with their own dark thoughts or past traumas, which may not necessarily be presented in a traditional horror style.
It's purely fictional. The plot and characters of The Enemy Within are the product of the author's imagination and not drawn from any actual incidents or people.