Tensei Majutsushi no Isekai Seikatsu: Gensaku Chishiki de Sekai Saikyou

Histoire

"The Adventure of Allen The Hero," is the novel written by our protagonist. Overwhelmingly popular and widely read, everyone was eagerly awaiting the conclusion of the story. Despite being told he doesn't have much time left to live, he put his life on the line to complete the series. Unfortunately, his lifespan ran out just as he finished writing the final volume— but… "Why did I become such a nobody character?" He unexpectedly transmigrated into the very novel he wrote. And he was put into the body of Will, an underdog mob character from the early part of the novel.

Synopsis via AniList (en anglais).