Tensei Majutsushi no Isekai Seikatsu: Gensaku Chishiki de Sekai Saikyou

"The Adventure of Hero Allen." The protagonist, whose days are numbered, is putting his life on the line as an author to complete his novel. An overwhelmingly popular work known to everyone. As readers around the world eagerly await for the final volume, his lifespan unfortunately ran out. But... "Why am I getting transmigrated into a mob character fated for ruin?" He unexpectedly got transmigrated into the novel he wrote. And he was transmigrated into Will, the underdog mob character from the early part of the novel. The eldest son of the soon to be ruined noble family, a character with no talent in magic whose future is fated to walk a miserable path. It was the worst starting point, but it means nothing for him, the original author. As the writer of the original work, he knows of all the quests and events that will happen, strong hidden items and magic, the characteristics and weaknesses of the monsters, and even the world's secret. In order to avoid getting doomed, Will shall use his knowledge of the original work and aim to be the strongest in the world. "I'll change the future. That's the reason why I have the knowledge of the original work." Using the original work knowledge cheat to change the fate of a mob that even left the hero in awe, he'll be an unparalleled character in this world!

(Source: HykeComic, translated)