Twin Sanity is purely imaginative. It's not drawn from real-life happenings. The story and characters are crafted from the author's creativity rather than real experiences.
Another benefit is the expansion of the Harry Potter universe. These fanfics can introduce new storylines that are related to the concept of sanity. Maybe it shows how the wizarding society as a whole deals with the idea of mental stability, which is not fully explored in the official books. It can also create new magical elements or spells that are associated with sanity.
Often in such fanfictions, the plot may involve a betrayal that pushes Harry over the edge. It could be a friend or a mentor who turns on him. This sudden shift in loyalty would shatter his world view. He has always relied on the support of those around him, and when that is taken away, it becomes difficult for him to stay sane. His actions would then become erratic as he struggles to come to terms with this new and cruel reality.
Finding the author would require some research. You could start by looking on popular 'Harry Potter' fanfic websites. Maybe check the tags or the description of the fanfic on those sites, which might give a clue about the author.
Since it's a fanfic, the possibilities are endless. It might explore the idea of peace and sanity in a post - Voldemort era. How the wizarding world rebuilds itself, how relationships are mended, and how the trauma from the war is healed all in the name of peace and sanity. It could also introduce new characters or expand on the backstories of existing ones in relation to these themes.
Maybe Orochimaru manages to get his hands on Naruto and subjects him to some dark and twisted experiments. These experiments could mess with Naruto's mind, causing him to lose his sanity. He could start seeing illusions all the time, believing his friends are his enemies. As a result, he'd be in a constant state of confusion and aggression, and his once - positive attitude towards life and ninja - hood would be completely gone.
Perhaps an unknown and powerful dark ninja. This ninja could be from a long - forgotten clan with dark powers. They might have a vendetta against the Leaf Village and see Naruto as the key to their revenge. So they use their dark arts to make Naruto lose his sanity and turn him against his own village.
It could be that Naruto discovers some dark secrets about his past, like his parents were actually killed by the very people he trusted in the village. This revelation is so shocking that it shatters his mental state. He becomes a completely different person, losing his sense of right and wrong, and his sanity along with it.
It could be Kaguya. If Kaguya were to return and use her powerful abilities to drive Naruto mad, perhaps by invading his mind with dark thoughts and memories. She has the power to warp reality and minds, so she could easily make Naruto lose his sanity. And since she has a grudge against the world of ninjas, making Naruto, one of the most powerful ninjas, go insane would be a great way for her to start her reign of terror all over again.
Obviously, Harry Potter himself is the central character. Since the story is about him losing his sanity, he is at the core of the plot. Then there would likely be his friends like Hermione and Ron. Their reactions to Harry's deteriorating mental state would be important aspects of the story. They might try to help him, or they could be at a loss about what to do.