Muzan Kibutsuji emerged over a thousand years ago in Japan’s Heian Era as a terminally ill human. A desperate experimental cure mutated him into the first demon, granting immortality, regeneration, and a bloodlust-driven existence. Obsessed with eradicating his vulnerability to sunlight, he relentlessly seeks the elusive Blue Spider Lily and engineers subordinate demons to cultivate sun resistance, intending to absorb successful specimens to claim their immunity. Pale-skinned with crimson eyes and a lean frame, he shapeshifts to evade capture, adopting guises like a woman with a chignon or a child. His body retains cellular scars from a near-fatal clash with Yoriichi Tsugikuni, the Demon Slayer who mastered Sun Breathing, leaving Muzan deeply traumatized and vengeful toward those who wield the technique. As progenitor of demons, he rules the Twelve Kizuki—Upper and Lower Ranks—through fear, exterminating underperforming subordinates. His narcissism fuels a contempt for imperfection and a belief in human inferiority, yet he strategically avoids direct conflict with the Demon Slayer Corps, manipulating events from shadows. A genetic link to the Ubuyashiki Family, leaders of his adversaries, underscores the conflict’s irony. Centuries prior, Yoriichi’s intervention spared Tanjiro Kamado’s ancestors, forging a feud. Muzan targets the Kamado lineage upon recognizing Yoriichi’s Hanafuda earrings, slaughtering Tanjiro’s family and turning Nezuko into a demon. In later battles, toxins and sunlight warp Muzan’s form: his hair elongates and whitens, his body ages rapidly, and gaping mouths sprout across his limbs. As dawn threatens his final stand, he morphs into a colossal infant-like entity to delay disintegration. Coordinated assaults from the Demon Slayer Corps, compounded by accelerated aging poisons and sunlight exposure, ultimately reduce him to ash.