The 4 Shocking Reasons Why Muzan Kibutsuji *Actually* Died In Demon Slayer

The question of who killed Muzan Kibutsuji, the King of Demons and the main antagonist of the Demon Slayer: Kimetsu no Yaiba series, is far more complex than a simple name. As of the latest updates in the manga and the ongoing anime adaptations in December 2025, the answer is not a single person, but a devastating combination of a secret weapon, the ultimate sacrifice of the Demon Slayer Corps, and the one weakness Muzan could never truly conquer: the sun. His defeat was a masterpiece of strategic warfare, a culmination of over a thousand years of history, and the ultimate price paid by humanity's greatest warriors.

Muzan's death marks the end of the Demon Slayer era, but the final battle itself was a brutal, all-night affair that saw the Demon King weakened, cornered, and ultimately destroyed by a relentless assault. This article breaks down the precise, multi-layered attack that finally brought down the seemingly immortal progenitor of all demons.

The Demon King's Profile: Muzan Kibutsuji Biography

Muzan Kibutsuji is the central figure of evil in the Demon Slayer universe, a being whose very existence caused centuries of suffering. His defeat required an understanding of his origins and his singular obsession: overcoming his greatest weakness.

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• Full Name: Muzan Kibutsuji (鬼舞辻 無惨)
• Title: Demon King, Progenitor of Demons, The First Demon
• Era of Birth: Heian period (circa 794 to 1185 AD)
• Origin: Born a sickly human who was predicted to die young. He consumed an experimental medicine, the Blue Spider Lily, which transformed him into the first Demon.
• Goal: To achieve true immortality by conquering his only weakness—sunlight—or by creating a demon who could walk in the sun (which he eventually attempted with Nezuko and then Tanjiro).
• Greatest Fear: Yoriichi Tsugikuni, the original user of Sun Breathing.
• Abilities: Immense strength, speed, regenerative abilities, shapeshifting, and the power to create and control all other demons through his blood.

The Master Strategy: Tamayo's Four-Stage Anti-Kibutsuji Drug

The true beginning of Muzan’s end was not a sword strike, but a meticulously crafted poison. Dr. Tamayo, a demon who had broken free of Muzan’s control, worked with Shinobu Kocho to develop a revolutionary, multi-faceted drug that would strip the Demon King of his powers. This was the critical factor that transformed the fight from an impossible battle into a survivable one.

The drug was introduced into Muzan's system when he absorbed Tamayo's body during the battle in the Infinity Castle. It was not a single poison, but four separate effects designed to systematically dismantle his demonic physiology.

1. The Human Conversion Drug

The first stage was intended to convert Muzan back into a human. While it did not fully succeed, it caused a massive, chaotic disruption to his cellular structure, forcing him to expend immense energy to combat it.

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2. The Accelerated Aging Potion

This was arguably the most devastating effect. The drug caused Muzan to age at an astronomical rate—equivalent to 9,000 years per hour. This rapid aging severely degraded his strength and speed, drastically slowing his regeneration and making him vulnerable to the Hashira's attacks. For a being who had existed for over a millennium, the sudden, extreme onset of mortality was a crippling psychological and physical blow.

3. Cell Division Prevention

Muzan's primary defense is his ability to rapidly divide and regenerate his cells. The third stage of the drug paralyzed this ability, meaning every wound inflicted by the Hashira and the Demon Slayers could not be instantly healed. This allowed the Demon Slayers to gain ground and inflict permanent damage.

4. The Cellular Destruction Component

The final, lethal stage of the drug was designed to break down Muzan's body at a cellular level, acting as a potent, internal poison. This effect, combined with the aging and paralysis of his regeneration, ensured that even his most desperate attempts to heal were futile.

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The Unwavering Sacrifice of the Hashira and the Corps

While Tamayo’s drug was the scalpel, the Demon Slayer Corps was the hammer. The final phase of the battle took place outside the collapsing Infinity Castle, forcing Muzan into the open just before dawn. The core mission of the remaining Hashira and Corps members was simple but suicidal: Hold Muzan until the sun rises.

Every surviving Hashira—Giyu Tomioka (Water), Sanemi Shinazugawa (Wind), Gyomei Himejima (Stone), and Obanai Iguro (Serpent)—fought with absolute disregard for their own lives.

• Gyomei Himejima: The Stone Hashira, widely considered the strongest, was instrumental in restraining Muzan, using his immense power to buy precious seconds.
• Obanai Iguro and Mitsuri Kanroji: The Serpent and Love Hashira were both brutally wounded, with Obanai even letting himself be bitten and slashed to prevent Muzan from escaping or regenerating.
• Giyu Tomioka and Sanemi Shinazugawa: They used their respective Breathing Styles to continuously pressure the Demon King, preventing him from focusing on escape.
• The Rank-and-File Slayers: Countless lower-ranked Demon Slayers willingly sacrificed their lives, forming human shields to protect the Hashira from Muzan's devastating attacks, embodying the ultimate spirit of the Corps.

The combined effort of the Demon Slayers, fueled by the memory of their fallen comrades and the desire to end the cycle of pain, was the physical constraint that locked Muzan in place. The drug had weakened him, but their sheer willpower kept him from escaping the inevitable.

Tanjiro Kamado: The Final Blow and the Rising Sun

The final moments of Muzan Kibutsuji’s life were a direct confrontation with his most hated enemy: the legacy of Yoriichi Tsugikuni. Tanjiro Kamado, after mastering the twelve forms of the Hinokami Kagura (Sun Breathing), was the only one capable of inflicting damage that truly resonated with Muzan’s ancient trauma.

As the sun began to peek over the horizon, casting its lethal rays onto the battlefield, Muzan, in a desperate, monstrous infant form, tried to burrow underground to escape. The remaining Demon Slayers struggled to hold him, but it was Tanjiro who delivered the decisive action.

Severely wounded and nearly dead, Tanjiro found a burst of strength and, using the final forms of the Sun Breathing technique, struck Muzan from within his own body, where he had been momentarily swallowed. This final, internal assault prevented Muzan from fully escaping the sunlight, holding him exposed just long enough.

The Final Truth: Muzan Kibutsuji was killed by the sun. However, the sun could only reach him because of the combined effort of the Demon Slayer Corps. The Hashira held him down, Tanjiro delivered the final, internal strike, and Tamayo’s four-stage drug weakened him to the point where he could not regenerate fast enough or move quickly enough to evade the dawn. It was a victory forged in science, sacrifice, and the ultimate power of the Sun Breathing legacy.