In this world, demons exist, but most of humanity doesn't know that demons are real. This is because demons can't go outside during the daytime, and they are pretty rare, but the humans that do know demons exist have the choice to become a demon slayer. A demon slayer kills demons as a job because demons eat humans. Demons need to eat humans for food. Tanjiro was with inosuke, zenitsu, and Tengen on a mission to kill a demon in the entertainment district. Demon slayers use katanas.