How Did Muzan Become a Demon in ‘Demon Slayer’?

how did muzan become a demon

Muzan Kibutsuji is the main antagonist of Demon Slayer. He is a Demon, being the first of his kind, who has lived for more than a thousand years, being as such, the one responsible for the origin of the other demons, responsible for having corrupted Michikatsu Tsukiguni and being the one who murdered Tanjirō’s family and Nezuko, turning the latter into a Demon, being the trigger for Tanjirō’s desire for revenge towards him. In this article, we are going to tell you how Muzan became the first Demon in Demon Slayer.

Throughout his young life, Muzan struggled with an illness that would have killed him before he reached the age of 20. The doctor treating him gave him a special prototype medicine, but it appeared to have failed, and Muzan killed him. Only after killing him did he realize that the medicine was working and had helped him acquire a strong body. What he did not know at that moment was that the medicine had turned him into a Demon.

The rest of this article will focus on Muzan Kibutsuji and his history, as we will explain how Muzan became a Demon in Demon Slayer. Muzan Kibutusji has a lot to offer in terms of history and origins, which is why we reckoned that he deserved proper treatment in a standalone article.

How did Muzan become a Demon?

To understand how Muzan became a Demon, we have to go way back to his birth and early history. Namely, when Muzan was in his mother’s womb, his heart stopped several times. They said he was stillborn, as he had no pulse and was not breathing. However, when he was taken to be cremated, he cried out for the first time.

When he was young, he was diagnosed with a disease that would kill him before he reached the age of 20. The attending physician gave Muzan a special prototype medicine, which seemingly did not work. Furious at his worsening condition, Muzan killed his doctor. It wasn’t until he killed the doctor that he realized the drug actually worked and had given him a strong body.

Muzan27s Child Form 28Anime29

He realized that he had to consume the flesh of humans to survive, but he didn’t mind. However, his new, more serious problem was his inability to walk in sunlight, as it burned him badly. That’s when he really regretted killing the doctor before he could finish his treatment with his medicine, the Blue Spider Lily. He looked for the flower all over the country but couldn’t find it.


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That’s why Muzan started creating other demons and planned to spread them around the world, hoping that one of them would find a way to defeat the sun or at least help him in his search for the plant. This desire to be in the sun again has become his only true goal, and he has spent hundreds of years pursuing this obsession.

When did Muzan become a Demon?

The exact year of Muzan’s birth and his transformation into a Demon is not known, but we do know that he was born and lived during the Heian era of Japan’s history. This was long ago, as the Heian era ran from 794 to 1185. This means that Muzan’s transformation happened at least 700 years before the plot of Demon Slayer, but it is probably around 800-900 years.

Who turned Muzan into a Demon?

As for who turned him into a Demon, we have explained that his doctor did it. The name of his doctor was never revealed, and we don’t know whether he knew that Muzan would become a Demon during the procedure. The doctor intended to cure Muzan of his illness and to save his life, which he did, but we don’t know whether he knew it would turn Muzan into a monster.

Why was Muzan turned into a Demon?

Now that we’ve explained everything about Muzan turning into a demon, we can also try to explain why it happened. Namely, Muzan was a sick young man, and he needed help. Seeing how he was born during the Heian era, it is understandable that the medicine of the time could not have provided him with proper care. Sure, a physician took care of him, but medicine at the time was simply not advanced enough to cure Muzan and his health issues. That is why his doctor resorted to an experimental cure, hoping it would help Muzan.


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He found a blue spider lily, made a cure, and administered the first dose to Muzan. And it actually helped him, although he realized that only after he had already killed the doctor. He was stronger, and he wasn’t sick anymore, but – as he did not complete his treatment, he had several weaknesses, including his weakness to sunlight.

It was never explained why the cure turned Muzan into a demon, but we know that he was the first Demon in history. Whether the blue spider lily does that to everyone is unknown, as the author never really explained it. It could be that it is a property of the flower and that everyone who consumed the cure would’ve been turned into a demon, although it is highly unlikely that the doctor knew that. It could also be that he turned into a demon because the treatment was incomplete, and had he consumed all the doses, he would’ve been a normal, healthy human.

It is also possible that the blue spider lily actually manifests a person’s deepest and darkest desires, and since Muzan’s soul was quite dark already, it simply gave him what he wanted. We don’t really know for sure, but one of these theories could, very well be correct.

Why did Muzan become evil?

To give you a precise answer to this question, we have to chronicle Muzan’s early history from when he turned into a demon to when he attacked the Kamado family. His endeavors in that period will give you the necessary insight to understand his evil turn.

Around the Oei era, Muzan found a young girl named Tamayo, who also had a life-threatening illness like him. She met Muzan and, wishing to see her children grow up, allowed him to transform her into a Demon. Unaccustomed to her new body, she inadvertently killed her own family so she could survive, as well as another group of people. This decision haunted her to no end, and she became resentful and hateful towards Muzan.

But due to Muzan’s immense power, she failed to successfully kill him at the time. Muzan then enslaved Tamayo, and she followed him everywhere like a humble servant. Soon after, he finds Michikatsu Tsugikuni, a Demon Slayer who had awakened the Mark of the Demon Slayer and a Moon Breathing user. Michikatsu wanted to become more powerful than his brother and hated the fact that his Mark meant that his life would soon be over.

Muzan proposed a compromise to him: he wanted to have a demon using a Breathing Style, and in exchange, Michikatsu could continue living on forever and perfect his technique. Michikatsu agreed, and Muzan gave him an extremely high concentration of his blood; three days later, Michikatsu returned, transformed into a Demon, and took the Demon name Kokushibo. Later, Muzan and Tamayo meet a demon slayer named Yoriichi Tsugikuni.


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As soon as they met, Yoriichi sensed Muzan’s malice and anger, comparing him to boiling magma. Simultaneously, Muzan couldn’t find anything in Yoriichi that looked like anything dangerous and didn’t fear his opponent. Already satisfied to have Kokushibo as a subordinate, Muzan attempted to kill Yoriichi without warning.

Yoriichi, however, was able to dodge the blows and discern Muzan’s weak spots – his seven hearts and five brains moving freely around his body. Yoriichi combined all of his forms into a single strike and turned his blade bright red, allowing him to cut through all of Muzan’s weaknesses and dominate him.

For the first time in his life, Muzan was knocked out, unable to heal, and sat in a pool of his blood and glared angrily at Yoriichi, ignoring his question of what he thought a life was worth. When Yoriichi stepped forward to kill him, Muzan used what was left of his energy and split his body in a powerful blast.

Although Yoriichi could destroy almost all of the fragments, the ones he missed escaped, allowing Muzan to regenerate again. This encounter deeply scarred Muzan, and with the help of Kokushibo, the pair slaughtered anyone who used the Sun Breathing Style, hoping to prevent another Demon Slayer who would have the power to kill him. They almost succeeded in completely exterminating this Breathing Style.

Muzan Kibutsuji threatening the Swamp Demon

Greatly fearing the man he believed to be the true demon, Muzan never directly confronted the Demon Slayers again and mostly sent demons in his place to do the job. He continued to fear Yoriichi even after his death.

When Muzan heard the news of a massacre involving the violent deaths of sixty-seven people, he went to the village where he met the culprit, an eighteen-year-old boy named Hakuji. He expressed his disappointment as he thought it was a Demon who had done the deed and not a human. Hakuji threatened him but was silenced by Muzan, who pierced his head with his hand.


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The Demon King spoke of his plan to form a faction of twelve powerful Demons and asked the young man if he was able to withstand the amount of blood given to him. Hakuji, broken by his loss and having nothing else left, did not resist and became the demon Akaza.

This story tells us the degrees of Muzan’s evil. Combined with his actual origin story, we ultimately find out that Muzan’s quest to become human once more, combined with the hatred he garnered for life itself, led him to become the monster that he is. It is a tragic backstory, as he did not become a Demon of his own free will, but an inexcusable tragedy because of the trail of blood Muzan has left behind him. This is, ultimately, the answer to this section’s question.

Why did Muzan turn Nezuko into a Demon?

During the Taisho era, Muzan was able to locate the Kamado family, who were the holders of the Sun Breathing Style. When Tanjiro Kamado was in town, Muzan attacked his family and injected his blood into them, hoping to create a demon that could withstand the sun.

However, he was disappointed when the family died from the poison of his blood, thinking that creating a Sun-resistant Demon wasn’t so easy after all. He left their house, seemingly unaware that there was a survivor, Nezuko Kamado, who later turned into a Demon. Muzan attacked and slaughtered the Kamado family in his search for a cure for his own condition, which is his tragic backstory.


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He was a villain and a murderer, but Muzan just wanted to be human again; he simply wanted to feel the sun once again and searched for a way to do it. Nezuko was just another in a line of his potential lifelines, but what he did not ultimately know is that she actually was what he wanted her to be.

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