Years ago when demon's first appeared they had a king that feared nobody and that king was named muzan but little did anyone know he did have his own fear and that was the first ever hashira who has sun breathing but there is another breathing style that is afraid of but that breathing style was lost in ancient time now skip years later were muzan was defeated by a demon slayer named tanjiro who has sun breathing and now 10 years later we have a new person who is 15 and he is about to become a demon slayer.