No. While 'Reign' is based on a true story, it takes many liberties for the sake of drama. It's a fictionalized account of Mary, Queen of Scots' reign, so not all events are portrayed exactly as they happened in history.
I'm not sure specifically as there could be many novels with this title or related to this phrase. It might be about a story where the protagonist is transported to another world ('isekai' means another world in Japanese), perhaps with adventures, magic, and new characters.
Writing fanfic that involves sexual content is generally not appropriate as it can violate ethical and moral boundaries, and may also go against the terms of use of certain platforms. Moreover, it objectifies the characters in an improper way.
The 'isekai kara kita yuusha novel' likely tells a story full of fantasy elements. The hero coming from another world is a common trope in isekai stories. He could be summoned there by some magic or accident. Once in this new world, he may have to adapt to different cultures, magic systems, and races. His experiences would form the core of the story, which might include battles against powerful foes, exploring unknown lands, and uncovering the secrets of this new world.
One common theme is friendship development. Since kita and bocchi are often characters in a story, their friendship might be explored in depth, showing how they get to know each other better over time.
The reign of the king appeared in the first episode of the anime, Little Fox Demon Matchmaker.
While waiting for the TV series, you can also click on the link below to read the original work of " Little Fox Demon Matchmaker " to understand the plot in advance!
In my opinion, 'Reign on Me' isn't a true story. It likely combines imaginative elements to craft an engaging narrative rather than being a direct account of real-life events. Maybe there are some common themes or emotions that can be related to real life, but not the entire story.