The first of these was Shin High School DxD, which ran for four volumes from 2018 to 2020. Even then, there were still a few spinoffs and continuations. Not counting the spinoffs, the series ran for 25 volumes, with the final one published in early 2018. Beforehand, Ishibumi wrote the novels Denpachi and Slash/Dog and began High School DxD two years afterward.
High School DxD began life as a light novel series by creator Ichiei Ishibumi and drawn by Miyama-Zero.
This actually helped it stand out from the crowd and before an incredibly popular franchise, most notably the anime version. Conversely, the series took its many times raunchy humor to levels that other popular shows at the time didn't.
After all, protagonist Issei is reincarnated and awakens to find that his whole world has changed, much like the protagonists of those anime who are reborn in similar circumstances. Even then, High School DxD somewhat laid the ground for that other narrative path to take over and was very similar to isekai fantasy. The series fits fairly well into the harem genre, which was once the premise and most ubiquitous genre in anime before isekai began to take over.
