The Butterfly and the Shadow (Shinobu x OC)

The Demon Slayer Corps, a military organization whose sole purpose is to battle demons led by the King of Demons, Muzan Kibutsuji. The battle between the Corps and the Demons have been running on and on for thousands of years.

During the Golden Age of the Demon Slayers, the Demon Slayer named Yoriichi Tsugikuni, known as the creator of breaths and the man who cornered Muzan and was even feared by him. Years passed and the battle with the demons went on, yet Yoriichi's will still lived on.

Now, a new Demon Slayer was born named Saito Inei, the Shadow Hashira. Who was later known not only to be the strongest pillar of the Taisho Era, but the strongest demon slayer who was said to be on par with Yoriichi Tsugikuni.