The librarian, who was the champion of the World Swordsmanship Championship, traveled to the world of Demon Slayer during the Warring States Period, awakened the Great Swordsman System, and became a disciple of the Jigoku brothers.

From then on, he created various legends of his own in this world!

Muzan Kibutsuji: Facing Yoriichi, I can still escape, facing the librarian, I have no choice but to die! Yoriichi Tsugikuni: Your strength has surpassed mine! Demon Slayer Corps: The librarian is the most powerful man!