Hard to say for sure. Simran could have some roots in reality, but it's probably mostly a fictional creation to entertain the audience and tell an engaging tale.
'Shouwu: The Thing of Misfortune' was published in Japan in 2013. The novel began with a mysterious murder case, and the protagonist, Kagami, gradually uncovered a huge conspiracy.
In the story, Kagami was an ordinary police detective who received a strange murder report. The victim was a young girl. After investigation, Kagami found that the case seemed to be related to a mysterious existence known as the "Shouwu".
In order to find this mysterious existence, Kagami began a dangerous investigation. He constantly encountered strange obstacles and traps, and at the same time, he discovered a series of secrets related to the case. As the investigation went deeper, Kagami gradually revealed the truth of the "Shouwu·Haunting Thing". This existence was an ancient demon that had been searching for a scepter that could control the entire world, and Kagami was its next target.
Under Kagami's efforts, the demon was finally defeated and peace was restored to the world. Through the complicated plot and mysterious atmosphere, the novel makes the readers feel the charm of a detective novel, and at the same time, it also probes into the profound problems of human nature, faith, and fate.
Simran is not based on a true story. It's a work of imagination crafted by the filmmakers to engage and entertain the audience. The plot and characters are invented to tell a compelling fictional narrative.
Since I'm not familiar with this 'hateful day novel', it's hard to determine the author. It might be a self - published work, in which case the author could be someone who is just starting out in the writing world and is trying to get their story out there. There are so many novels out there, and without more information, it's nearly impossible to say who wrote it.