After defeating Muzan, Tanjirou became a demon king. He can't be killed because the tremendous power he got from Muzan right before his dead. The power made him went out of control, so he tried to kill everyone around him.. including Kanao, Zenitsu, Inosuke, and others as well. Seeing his little sister, Nezuko in bleed, defending Zenitsu from his tremendous claws, Tanjiro gain some of his sense. Right after that, he immediately ran away leaving everything behind. His friends, his only family, his mentor, comrades, including himself. He got no way back after what he had done, so Tanjiro became a demon fugitive. This the continued story from the original one, a story where everythings becomes pitch black for Tanjirou.