Muzan Kibutsuji is the source of all demons and the ultimate evil behind every calamity in Demon Slayer: Kimetsu no Yaiba. Born over a thousand years ago in the Heian period, he was frail and bedridden from childhood, his life hanging by a thread. A kind physician saved him with a miraculous medicine, yet Muzan, impatient with the medicine's slow effect, repaid kindness with cruelty and cut the doctor down with his own hand. Little did he know that the medicine had already begun to work, granting him a nearly immortal regenerative power and a body of extraordinary might—but it also planted an irreparable flaw: a fatal fear of sunlight. From that day on, his obsession with a "perfect, eternal life" became a callous pursuit that burned across a thousand years. Muzan is the ultimate embodiment of self-interest, a cold-blooded tyrant who walks alone over mountains of corpses. He regards all living beings as nothing more than straw dogs, devoid of pity or remorse; the only force that drives him is the abyssal terror of death buried deep in his soul. As the progenitor of all demons, he wields nearly invincible power: he is undaunted by decapitation from Nichirin swords and immune to ordinary blades—his sole fear is the touch of sunlight. His blood carries a curse-like power that can turn any mortal into a demon servant at will, or drive them to self-destruct and perish with a single thought. His forms are ever-changing: now a handsome man, now a bewitching woman, now an innocent child; but his true, fearsome battle form is an abyssal monstrosity bristling with fanged maws and deadly whipping tendrils. Between Muzan and the protagonist Tanjiro Kamado lies a blood grudge as deep as the sea. Muzan slaughtered Tanjiro's entire family and forced his sister Nezuko down the demon's path—an act that ignited the boy's flame of vengeance, spurring him to join the Demon Slayer Corps and swear to slay demons. In the final, brutal battle, the Demon Slayer Corps paid in flesh and blood, wave after wave pressing forward, until at last, as dawn approached, they dragged the dying Muzan out of the shadows and exposed him to the light of day. As the morning sun rose, that sin-laden body which had run rampant for a thousand years was finally burned to ash in the blazing radiance, consigned to eternal nothingness.
"In all these thousand years, I have never seen a god or a buddha. I have never been struck by any heavenly punishment."