It's not based on a true story. The plot and characters of 'The Devil You Know' are the result of the author's imagination and creative writing. There are no real-life counterparts for the events and individuals depicted in it.
Well, it might be a story that is based on a real - life encounter with a difficult or 'devilish' person or situation. For example, in a workplace, there could be a toxic coworker. Instead of trying to change the situation or find a new job right away, people sometimes stick with it because they know what to expect from this 'devil' they are familiar with. It could be exploring that kind of dynamic in a true - story context.
It could be about many things. Maybe it's about a real - life encounter with a difficult or malevolent person. It might be based on a true crime situation where someone had to deal with a 'devil - like' figure in their community or personal life.