500 years before the events of Demon Slayer, Muzan Kibutsuji escapes a gruesome death at the hands of Yoriichi Tsugikuni, the first Sun Breather and the inventor of all breathing styles. While Yoriichi failed to kill Muzan, the progenitor of all demons developed an intense fear of Yoriichi and Sun Breathing, which shaped the course of history in Demon Slayer.