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.
I'm not sure specifically which 'devil you know real story' you're referring to. It could be a personal account, a piece of folklore, or something related to a specific event or situation. Without more context, it's hard to say exactly what it's about.
I'm not sure specifically as there could be many different interpretations in Twilight fanfiction. It might be about a character facing a known 'devil' - perhaps a familiar antagonist in the Twilight universe in a new and interesting scenario.
The 'the devil you know real story' might be related to the concept that sometimes it's easier to tolerate a known evil or difficult situation rather than venture into the unknown. For example, in some workplaces, employees might put up with a bad - tempered but predictable boss because they fear the uncertainty of a new job or a new boss who could potentially be worse. It could also be a story in a horror or thriller context, where a character is haunted by a familiar, perhaps demonic, presence that they have some history with.