Muzan Kibutsuji, born a terminally ill human in Japan’s Heian era, cheated death moments before cremation after being declared stillborn, foreshadowing his relentless battle against mortality. Facing a predicted demise at twenty, he underwent a radical procedure using the elusive Blue Spider Lily. Enraged by its delayed effects, he murdered the physician, only to belatedly discover the treatment had granted him immortality, superhuman abilities, and a fatal sunlight vulnerability—transforming him into the first demon. Obsessed with eradicating his weakness, he pursued perfection across centuries. For over a millennium, Muzan hunted the Blue Spider Lily to conquer sunlight resistance. Failing to find it, he spawned legions of demons, seeking one who might inherit immunity. The Twelve Kizuki emerged as his elite enforcers. He commanded most demons through blood-bonded control, though exceptions like Nezuko Kamado and Tamayo defied his dominion. Pale-skinned with curly black hair, plum-red feline eyes, and sharp blue nails, Muzan frequently shifts appearances—masquerading as a refined woman in a black yukata, a frail child, or other guises. After regenerating from a flesh cocoon, his hair whitens, limbs sprout fanged, crimson masses, and cellular-level scars from his near-fatal clash with Yoriichi Tsugikuni resurface when weakened. Narcissistic and merciless, Muzan equates change with decay, clinging to permanence. He annihilates subordinates for perceived failures, dissolving the Lower Ranks after Rui’s defeat. Though outwardly composed, he erupts into violence when provoked, exemplified by slaughtering a drunkard who mocked his pallor. His interactions betray insecurities over his human fragility and obsession with eradicating vulnerability. Biologically aberrant, Muzan wields seven hearts, five brains, and regeneration defying decapitation. Sunlight and poisoned Nichirin blades remain his sole weaknesses. His Blood Demon Art enables cellular manipulation—shapeshifting, rapid adaptation, and demon creation. He weaponizes his blood, injecting lethal doses to destroy foes internally. Pivotal events include slaughtering the Kamado family, transforming Nezuko, and battling Tanjiro. In his final hours, Tamayo and Shinobu’s poison ages him 9,000 years, forcing a grotesque infant form to stall sunlight disintegration. Cornered, he transfers his blood to Tanjiro in a failed bid for succession. Muzan’s death unravels all demons, ending his eternal reign. His legacy intertwines with the Ubuyashiki bloodline and Kokushibo, a former slayer turned Upper Rank One. Centuries later, Muzan’s dread of Yoriichi Tsugikuni—the sole adversary to nearly kill him—exposes the fragility beneath his facade of invincibility.