Demon Slayer: How did Muzan become a demon? Explained

Muzan Kibutsuji is the primary antagonist of the Demon Slayer series. A progenitor of all demons in existense, Muzan's reign of terror began in Heian Era, lasting for a thousand years. During this time, his goal was to become immortal and live without the fear of death.

To accomplish this, he has spent his time finding the Blue Spider Lily and creating more demons, hoping one day they'll conquer their weakness of sunlight.

Interestingly, Muzan wasn't born a demon. He was turned into one thousand years ago, leading to his journey of trying to live forever.

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Demon Slayer: The way Muzan became the first demon

Muzan as seen in the Demon Slayer manga (Image via Shueisha)

Born in the Heian era of Japan, Muzan Kibutsuji bore the weight of delicate health, an unavoidable curse that afflicted him even before his birth. While he was in his mother's womb, his heart stopped multiple times. Upon entering this world for the first time, he was pronounced dead as he wasn't breathing and had no pulse.

Deemed a stillborn, he was then taken to be cremated, but he managed to escape death by crying for the first time. However, he struggled with illness throughout his young life and was later diagnosed with a terminal illness that would kill him before he turns 20.

Muzan killed his doctor in Demon Slayer (Image via Ufotable)

Muzan was desperate to stay alive, so he allowed the generous doctor treating him to give him an experimental medicine. The doctor treated him with a medicine that was made from a flower known as the Blue Spider Lily. Despite the doctor's efforts, Muzan's condition started worsen and he assumed the experiment had failed.

Enraged by his worsening conditions, Muzan killed the doctor. However, only after his death, Muzan realized that the medicine was working and had given him some extremely powerful set of attributes such as inhuman strength and immortality. Soon he realized that he would need to consume human flesh to survive, but he had no issue killing humans to satiate this hunger.

Muzan couldn't walk in sunlight (Image via Shueisha)

However, a bigger problem arose infront of Muzan - he realized he couldn't walk in the sunlight without getting burnt. Thus, he once again was limited by another weakness, confining him to a life of darkness. This made him regret murdering the doctor before he could finish his treatment with the Blue Spider Lily.

In his quest to achieve total immortality, Muzan began searching for the flower but was unable to find it anywhere in Japan. So, he created more demons hoping that one of them would conquer their weakness of sunlight or at least help him search for the Blue Spider Lily.

Thus, the world of Demon Slayer became a nightmare as Muzan gave his blood to humans, transforming them into powerful demons. If a significant amount of his blood is consumed at once, it can causes mutations that lead to death. However, in small doses, it can make a demon really strong.

In the Heian Era, the Demon Slayer Corp was founded to shield humanity from demons. The organization received a significant reinforcement in its ranks during the Sengoku Era, coming from the exceptional skills of a talented swordsman, Yoriichi Tsugikuni. His Sun Breathing Style not only contributed to the corps' strength but also laid the groundwork for the emergence of various other breathing styles.

Yoriichi came close to defeating Muzan, prompting the demon to hide for a century.

Tanjiro assumed Yoriichi's matle, and through the combined efforts of both himself and the Demon Slayer Corps, they successfully brought an end to Muzan and his thousand-year reign of terror.

Muzan Kibutsuji was a cold-hearted and ruthless antagonist whose weakness was the catalyst for the entire Demon Slayer series. In his quest for immortality and his fear death, he destroyed thousands of lives and lived a wretched existence full of misery and bitterness.