Tensei Majutsushi no Isekai Seikatsu: Gensaku Chishiki de Sekai Saikyou

"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. The eldest son of a noble family soon to fall into ruin, he's a character with no talent in magic, with a miserable future ahead of him. It was the worst starting point, but that's no problem for him, the author. As the writer of the original work, he knows all when the quests and events occur, where the strongest magical items are hidden, all the characteristics and weaknesses of the monsters, and the secrets of the world itself.

In order to change his doomed fate, Will will use his knowledge of the original story and aim to become the strongest in the world. "I’ll change the future. That’s the reason why I have all this knowledge." Using his story knowledge as a cheat to change the fate of a mob character that will even have the hero in awe, he’ll gain unparalleled power in this world—!

(Source: Hyke Comic, translated)