Prologue

Kimetsu no Yaiba is owned by Koyoharu Gotōge.

Tanjiro and Kanao were in the same mission in battle. Unexpectedly, tragedy had come to face them. The infamous pillar Tanjirou was demonized and now had to obey the demon leader's orders.
Promise broken.

Warning: SPOILER (don't read if you haven't read the manga yet)
Expected OOC.

Reference from a Doujinshi.