How Muzan Became A Demon, Explained

Demon Slayer is a historical fantasy anime set in Japan’s Taisho Era, in a reimagined world where humans live alongside demons. These demons are incredibly dangerous, and are such a problem that the Demon Slayer Corps exists to fight them off and try to protect humanity. The various demons in the world have different powers and different levels of strength, but they all have the same weakness that will kill them – sunlight. If those fighting them are able to keep them exposed until sunrise, it is the only surefire way to defeat even the strongest demons.

The demons all had to come from somewhere, and in the world of Demon Slayer, that is from the first demon, Kibutsuji Muzan. Muzan is the main antagonist of the story, and the original source of all the demons that plague the world. As such, Muzan is known as the Demon King, and the leader of the Twelve Kizuki, the most powerful demons in the world. But Muzan himself also had to come from somewhere; he was not born a demon, and didn’t just randomly appear. So how did Kibutsuji Muzan become a demon?

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Who is Kibutsuji Muzan?

The Man Who Feared Death

When the reader or viewer is first introduced to Kibutsuji Muzan in Demon Slayer, either through the manga or the anime, he appears to be a man without any weakness. He is fashionable, handsome, and charismatic on one hand – and ruthless, terrifying, and deadly on the other. He is also the one responsible for killing Tanjiro’s family and turning Nezuko into a demon, which makes him the focus of Tanjiro’s revenge. Muzan is also the Demon King, and as the source of all demons, as long as he exists the demon plague will never end.

As the story of Demon Slayer unfolds, though, there is more to Muzan than meets the eye. Despite his ruthless character and violent nature, he also has a serious emotional weakness – he fears death. Muzan’s goals are all centered on achieving true immortality, and overcoming his (and all demons’) weakness to sunlight. While nothing else seems capable of stopping him because of his incredible power, Muzan knows that he has the same weakness to sunlight that all demons have. It restricts his movements in the daytime, and it gives the Demon Corps a way to defeat him.

Muzan is motivated by an extreme desire to live. All of his plans and everything he does is motivated by his desire to overcome death, and to live without the human fear of dying. In Demon Slayer Chapter 98, Muzan says to the demon Gyokko;

Muzan wants to become “eternal,” and enter this perfect state. But he was not always a nearly immortal demon, and started with the same human weaknesses as everyone.

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Was Muzan a Human?

How Did He Become a Demon?

Kibutsuji Muzan began his life as a human – but during Japan’s Heian Era. To put that into context, the Heian Era was from 794 to 1185. Demon Slayer is set in the Taisho Era, 1912 to 1926. In Demon Slayer, Muzan has already been a demon for more than a millennium, and the demons he created have been terrorizing the world for hundreds of years at his bidding.

Muzan did not have an easy start to his life, and was fighting against death even while he was still in his mother’s womb. In his final moments before being defeated by the sun after fighting with Tanjiro, Muzan revealed that before he was even born, his heart would stop inside his mother. When he was born, his heart stopped, and he was declared a stillborn. Only through his immense will to live was he able to start breathing before he was cremated.

That was only the start of Muzan’s struggle against death, though. Chapter 127 of the Demon Slayer manga shows what happened to him. Muzan became terminally ill as a teenager, and by age 19 it seemed likely he would not live to see 20. A doctor tried to help him, though, not giving up on Muzan’s case. The doctor created a special medicine from a blue spider lily, but Muzan did not understand the “gift” he had been given until it was too late. Thinking that he would not live much longer, and frustrated that he had been fighting against death his whole life only to lose the battle, Muzan killed the doctor in his anger.

However, Muzan realized shortly after that the doctor had “cured” him, in a sense; the medicine from the blue spider lily had made him stronger, and taken away his disease. Demon Slayer Chapter 127 reads;

What it had left him with was a weakness to sunlight, and a desire to eat humans. He had become the first demon. His weakness to sunlight meant that he was still not immortal, though, and did not have a perfect body, which is what drove him to try to unlock the secrets of the blue spider lily. With the doctor dead, though, Muzan was not able to replicate or perfect the medicine, or even how to cultivate the flower.

Muzan’s fear of death and desire for an immortal, perfect body that had no weakness is what drove him to seek out Nezuko so vehemently in the latter parts of Demon Slayer. When he learned that she had conquered the sun despite being a demon, Muzan became obsessed with finding her and learning how she had done what he had not been able to in a millennium. Muzan became a demon thanks to a doctor trying to cure him and keep him from dying, and while it did prolong his lifespan far beyond that of any human, his weakness to sunlight would eventually kill him in the end.

Watch Demon Slayer now on Crunchyroll.

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