The story of the Demon King Muzan Kibutsuji from Demon Slayer: Kimetsu no Yaiba.
Past[]
As a human[]
Muzan Kibutsuji was born during the Heian Era of ancient Japan to the noble Ubuyashiki family and suffered greatly during his life as a human from his extremely weak and frail body. His heart stopped multiple times when he was still a fetus in his mother's womb and he was declared stillborn upon birth, as he wasn't breathing and had no pulse. When he was eventually taken away to be cremated, he narrowly escaped death by releasing his first cries.
During his childhood, Muzan was constantly sick; as a young man, he was diagnosed with a rare unnamed disease that would kill him before his 20th birthday. When he was still 19 years old, a doctor tried to treat him by giving him a special experimental medicine, but when it seemed like it was only making his condition worse, Muzan came to believe that the doctor was a fraud and killed him out of rage by stabbing him in the head.
Muzan with his doctor
Becoming the Demon King[]
However, not long after he'd taken the medicine, Muzan realized that his physical constitution had improved tremendously and that the doctor's treatment was indeed working. It also caused him to develop a craving for human flesh and he began consuming humans without hesitation to satisfy his new appetite, with his family members being his first victims. As time passed, Muzan quickly adapted to his new life as a demon and was thrilled with the all the incredible abilities that the doctor's medicine had given him. However, he would later discover that being cured of his illness had left him with a serious problem; he could no longer walk in the sun and if he tried, it would burn him severely, even realizing that it would kill and turn him into ashes.
While Muzan was glad that he no longer had to worry about dying from disease, the thought of having to spend the rest of his life avoiding sunlight during the day and only being able to go out at night infuriated him and he came to truly regret his foolish decision to kill the doctor before the latter had finished treating him. Upon conducting research on the contents of the medicine that the doctor had made for him, he discovered that it contained a rare flower called the Blue Spider Lily and believed that it could nullify his weakness to sunlight if he consumed it. To that end, he spent centuries searching for it all over Japan to no avail, as he was unaware that it only bloomed in the daytime for at most 2-3 days a year.
Even though he had obtained immense power, Muzan considered himself to be imperfect and wanted to somehow get rid of his weakness to sunlight. This desire eventually became an obsession that he would chase for the next 1000 years and, with the search for the Blue Spider Lily going nowhere, he reluctantly decided to use his blood to create more demons under his control, hoping that one of them would eventually conquer the sun; once this happened, he could take their immunity to sunlight for himself by consuming them and then finally achieve his goal of becoming truly immortal.
It was around this period of time that Muzan married and settled down with five different women as his wives while still posing as a human. However, due to his complete lack of compassion and humanity as a demon, he ended up abusing them to the point that they all committed suicide.
At some point during the first few centuries of his lifetime, presumably during the Kamakura or Muromachi Era, Muzan met a young woman named Tamayo, who was sick with a life-threatening disease and was desperately looking for a cure so that she could get the chance to see her children grow up into adulthood. Hiding his true intentions, Muzan offered to treat her illness and Tamayo accepted, unaware of the consequences of this "treatment" that he was about to give her.
Upon being transformed into a demon, Tamayo developed an insatiable thirst for blood and went on a mindless rampage around town, massacring not only her husband and children, but also many other innocent people. After regaining her sanity and realizing what she'd done, Tamayo was struck with immense guilt and attempted to kill Muzan as revenge for tricking her into killing her own family. However, due to the massive difference in power between them, Tamayo failed to even hurt him and the realization that she would never be able to kill him on her own completely broke her spirit, leading her to adopt a much more docile attitude towards him and start following him around as his unwilling slave.
During the Sengoku Era, Muzan learned of the Demon Slayers Corps, an army of human warriors formed by his relatives in the Ubuyashiki family that opposed him and his minions, and encountered a marked Demon Slayer named Michikatsu Tsugikuni, who used a Breathing Style called Moon Breathing. Due to having seen it firsthand with his comrades in the Demon Slayer Corps, Michikatsu was aware that the curse of the mark would kill him within the next few years and hated it more then anything, as it would rob him of his chance to achieve his life's goal of surpassing his twin brother in strength.
Seeing an opportunity here and desiring to have a demon who could use Breathing Styles under his command, Muzan offered Michikatsu the chance to become a demon, as it would make him immortal and give him all the time he needed to perfect his fighting skills and become stronger than his brother. Michikatsu agreed and Muzan responded in turn by injecting him with an extremely high concentration of his blood. Although it took three days for the transformation to complete itself, Michikatsu ultimately survived the process and took on the name Kokushibo as a way of discarding his past as a human.
Not long after this event, Muzan encountered Yoriichi Tsugikuni, Michikatsu's younger twin brother and a marked Demon Slayer who used a Breathing Style called Sun Breathing, and tried to kill him without warning. However, not only did Yoriichi easily dodge his attacks, but he also turned his sword bright red and overpowered Muzan in one strike. For the first time in his life, Muzan had been beaten, unable to heal from his wounds, and when Yoriichi moved forward to kill him with his second strike, Muzan split his body up into hundreds of pieces and fled. Although Yoriichi was able to destroy most of the fragments with his attacks, the pieces he missed escaped and allowed Muzan to regenerate his entire body elsewhere. However, due to having expended so much power to escape, Muzan unintentionally released his control over Tamayo, who witnessed his near-defeat against Yoriichi and took this opportunity to escape herself after Yoriichi realized that she meant him no harm and subsequently spared her life.
Muzan was deeply scarred by this experience and with Kokushibo's assistance, spent the next 500 years murdering anyone who knew how to use Sun Breathing in order to prevent the rise of another Demon Slayer of Yoriichi's caliber. His deep fear of Yoriichi, who he considered to be the "real demon," also led him to never directly confront the Demon Slayers again and instead send his subordinates to do his bidding instead. Even centuries after Yoriichi died, Muzan still remained afraid of him and his overwhelming power.
In time, Muzan came to form the Twelve Kizuki, an organization made up of the twelve strongest demons under his control, and split them into two groups, the six Upper Ranks and the six Lower Ranks. While the Upper Ranks remained unchanged for over a century and were always the ones disposing of the Hashira, the strongest warriors of the Demon Slayer Corps, the Lower Ranks proved to be far weaker and were constantly getting themselves killed by the Hashira once every few years, much to his growing disappointment. Among the Twelve Kizuki, those who he personally transformed into demons aside from Kokushibo included Doma, Akaza, Hantengu, Gyokko, Nakime, Enmu, and Rui.
During the Taisho Era, Muzan was able to locate the Kamado family, a clan of coal burners and the last users of Sun Breathing, up in the mountains of remote Japan and injected them all with a massive amount of his blood on a whim, hoping to turn them into demons that could withstand sunlight. Much to his disappointment, they all died in the process and he left their corpses behind in the ruins of their family home, unaware that Nezuko, the eldest daughter of the family, had survived and that Tanjiro, the eldest son of the family and the only one who actually knew how to use Sun Breathing, had been away from home at the time of his attack. Upon returning home the next morning, Tanjiro stumbled across the bodies of both his mother and his younger siblings, and quickly deduced that they'd been attacked by a demon while he was away. After realizing that Nezuko was still alive, albeit as a demon, Tanjiro joined the Demon Slayer Corps in order to find a way to turn her back into a human and get payback on Muzan for slaughtering his family.
Demon Slayer: Kimetsu no Yaiba[]
Introduction[]
Muzan encountering Tanjiro for the first time.
Eventually, Tanjiro Kamado coincidentally picked up Muzan's scent and approached him on an urban street. At first, Muzan didn't know who Tanjiro was until he saw a pair of earrings on Tanjiro's ears - earrings worn by swordsmen who use Breath of Sun swordsmanship: the same swordsmanship technique that once drove Muzan to the brink of death. But since he was walking with his (human step-)daughter, he put up a kind facade and pretended to wonder who Tanjiro was. Tanjiro was mad at him and wanted to kill him. In order to get away and distract Tanjiro, Muzan sliced a random passerby's neck and turned him into a demon. While that demon attacked his wife, Muzan got away with his family. Before entering the carriage, he told his wife he had to do something and that they should go ahead.
He then walked through an alleyway where a drunk guy bumped into him. The drunk started mocking him and stated he didn't like fancy-pants like him. When the drunk mocked him for his pale skin, Muzan smashed him into the wall, killing him instantly. The muscular brother of the drunk then angrily attacked him, only to be kicked into the air and then killed. The drunk's girlfriend held his corpse, crying. Muzan then rhetorically asked her if he looked pale or weak, then replied to himself, stating he was infinitely close to perfection. He then stuck his finger into her forehead. Her body received so much blood from him that it couldn't handle the demon transformation, causing her to melt away and die in a gruesome manner. Muzan then summoned his subordinates, Susamaru and Yahaba, ordering them to kill Tanjiro: the Demon Hunter who seemed to have a connection to the dreaded swordsman who used the deadly Breath of the Sun - the swordsman who was the only human who came close to killing the Demon King.
Muzan's Frustrations[]
Disbanding the Lower Ranks[]
Muzan in a female disguise.
After the demise of Kyogai and later Rui, the Lower Rank Five, Muzan was frustrated by the Lower Ranks' constant failures. He called the Lower Rank One, Two, Three, Four and the current Six to a meeting, rebuking them and believing he no longer needed the Lower Ranks. As a result, Muzan started to kill the Two, Three, Four and Six one by one, as they respectively were begging for Muzan's blood, trying to flee, trying to deny Muzan's opinion, and trying to think about flattering his master. Enmu, the Lower Rank One, was the only Lower Rank left. However, unlike the others, Enmu showed no fear of death and even nonchalantly accepted the prospect for pure excitement. Enmu's sadistic streak impressed Muzan, so he sent Enmu on a mission instead. However, it's heavily implied that he somewhat knew Enmu may not succeed in his mission, so he sent Akaza to take care of Tanjiro should Enmu fail.
Some time later on, Muzan (living once again another human life as a young boy named Toshikuni, who pretends to have a skin condition) receives a report from Akaza that while he failed to find the Blue Spider Lily flower, he managed to kill the Flame Hashira Kyojuro Rengoku. However, Muzan is not pleased by Akaza's report and proceeds to torture him, lecturing that he allowed three other Demon Slayers (Tanjiro, Zenitsu, and Inosuke) who weren't even Hashira to live, additionally berating the Upper Three Demon for not finding any information on the flower. After briefly insulting him, Muzan sends Akaza away. Then in the Entertainment District of Yoshiwara, Muzan visits Daki, the Upper Six of the Twelve Kizuki. He then goes on to praise her, hoping that she will become stronger and crueler than before. He then tells her about Nezuko and proceeds on giving her the task to kill her.
The Upper Rank Meeting[]
Muzan reporting the Upper Ranks of Gyutaro and Daki's deaths.
Following the death of both Gyutaro (who was the true Upper Six) and Daki, Muzan proceeds to gather all of the remaining Upper Ranks in the Infinity Castle. There, Muzan is experimenting, hoping to find something to heal his weakness to the sun. After gathering the Upper Ranks, he informs them of Gyutaro and Daki's death, where he states that he regretted putting confidence in Daki and wished that Gyutaro simply poisoned the Demon Slayers so he would've succeeded. He then denounces the Upper Ranks, claiming that that they are no longer ranked by power anymore but by their remaining humanity, using Gyutaro's love and care for Daki as an example.
When Doma (in his attempt to placate his master) tried to ask Muzan if he had ever failed him, Muzan responded by citing their failures on not only finding the Ubuyashiki Family, but not also finding the Blue Spider Lily flower. After a brief pause, Muzan slams his hands on the table, before angrily stating that he is unsure why they are still existing, causing his chemistry equipment to shatter in his boiling frustrations. Hearing and sensing his anger, the Upper Ranks each gave a response; Hantengu grows more fearful and begs for forgiveness, Akaza remains silent knowing his recent failure regarding the Mugen Train mission, Kokushibo humbly accepting his inability to find the Ubuyashiki Family, and Doma somewhat apologizes for his failure. Gyokko however speaks about having new information to present, causing him to suddenly appear in Muzan's hand as a severed head without even realizing it. While he reads Gyokko's mind for his information, Muzan speaks about his detest towards change in any form, be it in situation, body, or emotion, then angrily demanding the Upper Rank Five to never present his information without confirming it, before dropping the demon's head. Enraged at the first change in the Upper Ranks in 113 years, Muzan demands the Upper Ranks to all exert and push themselves with greater desperation, before ordering both Hantengu and Gyokko on a new mission once the information is confirmed. Afterwards, the doors summoned by Nakime closes on Muzan, ending the meeting.
Muzan thrilled after finding out Nezuko is immune to the sun.
However one day, he felt that Nezuko was able to walk in the sun, which was also confirmed by Hantengu, who scouted out before dying. He was so thrilled to consume her and finally reach perfection, that he immediately killed the family he was hiding with. He then remembers back when he first became a demon and how he was determined to find a way to overcome the sunlight and become the immortal being he always dreamed of.
The Final Showdown[]
The Final War[]
Thanks to Nakime's infiltration and finding the HQ of the Demon Slayer Corps, Muzan arrived at the Demon Slayer Estate and confronted their leader, Kagaya. Kagaya predicted this, however, and sacrificed himself by placing bombs. Kagaya died and Muzan, heavily injured, was left open for Tamayo to inject a drug that reverts demons to humans into him. He is further endangered when Himejima attacks and manages to destroy his head, but Muzan is revealed to be able to regenerate it and the demon slayers realize the only way to kill him is through sunlight. Before he can be overwhelmed by the arriving Hashira, everyone is transported to the Infinity Fortress, an infinite fortress labyrinth controlled by Nakime. The demon slayers are scattered while Muzan takes time to heal, absorb Tamayo, and resist the drug.
While the Demon Slayers fought and Muzan healed inside of a fleshy cocoon, all of his Ranks were killed, except Nakime. He eventually overcomes the drug and emerges fully healed and stronger than ever. Dozens of Slayers surrounded him, but he simply killed each of them and only grew stronger with each one he ate. Eventually, he confronted Tanjiro, who told him that he should no longer exist. Muzan was tired of them wanting to avenge their loved ones. In his eyes, they should see themselves as lucky and move on with their lives. Muzan told them that they should see his actions as a natural disaster, comparing his slaughtering to earthquakes or storms. However, more Hashria arrive and Yushiro is revealed to have taken control of Nakime and Muzan is forced to remotely kill her to avoid having Nakime's ability used against him, but Yushiro is able to use the last seconds of Nakime's life to have her force the entire castle above ground. With Muzan exposed to the outside world, the demon slayer corps launch a desperate effort to contain him until sunrise. Despite being heavily outnumbers, Muzan is more than a match for the Demon Slayers, easily mutilating the Hasira and infecting them with his poisonous blood. However, they keep fighting and the tides start to turn when Yushiro provides an antidote to Muzan's blood poison blood and the Demon Slayers discover that bright red blades hinder his healing. Eventually, only Tanjiro is left standing, and he fights back with the secret 13th technique of Sun Breathing, a technique specifically designed to defeat Muzan. Tanjiro unable to use the technique as powerfully as the original creature, but Muzan still struggles against it and realizes something is hindering him. He scans the memories of Tamayo's cells and discovers the humanizing drug was only part of what she injected into him. She also added three addition drugs, one that prevents him from splitting apart and escaping like the first time he was nearly killed, and second that has him to rapidly age over 9,000 years, and a third designed the poison and destroy Muzan's cells. With his rapid aging weakening him, Muzan is left vulnerable to Tanjiro's technique until the latter's body reaches his limit. Before Muzan can kill Tanjiro, the latter is saved by Obani, who was treated by Yushiro, and the two continue the fight against the demon king. With the drugs, wounds, and long battle taking their toll, exposing the original wounds left by Yoriichi, which have never properly healed. Unable to separate himself and escape, Muzan becomes desperate and grows on a mouth on his torso to unleash a sonic scream, stunning his enemies. However, Insokue and Zenitsu rejoin the fight and work with Tanjiro to stall Muzan for as long as possible. Tanjiro manages to pin Muzan to a building and the sun starts rising, causing Muzan panics and tries to force his way to safety, but Tanjiro refuse to let him escape. To protect himself, Muzan forms a massive baby-like mound of flesh around himself to buy time which swallows up Tanjiro. However, the rest of the demon slayer corp do everything in their power to keep him in the sunlight. With some help from the surviving hashria, Muzan is further hindered and forced to start digging underground to escape the sun. Just before he can escape, Tanjiro lands a vital blow from inside Muzan's flesh which stalls Muzan long enough for the sun to finish burning him to ash. After over a thousand years, the demon king has finally died, but his evil has yet to be fully purged...
The Last Dice[]
Although Muzan had finally been killed, it came at the cost of the lives of Gyomei, Mitsuri, and Obanai, along with countless others who had sacrificed themselves to stall the Demon King long enough for the sun to rise. Before his physical death, Muzan was also able to transfer both his blood and his consciousness into Tanjiro's dying body, reviving the latter as the next Demon King who is immune to the sun and leading him to mindlessly attack his allies. He then attempted to brainwash Tanjiro into carrying out his dream and living forever by remaining young and immortal, but the latter rejected everything he had to offer and with the help of Kanao, Nezuko, Inosuke, and Zenitsu, was able to successfully revert back into a human using the demon-curing drug, destroying Muzan's consciousness for good and putting an end to his 1000-year reign of terror once and for all.
Beyond the Sanzu River[]
In the spin-off bonus chapter that deals with the immediate aftermath of the final battle against Muzan and Tanjiro's subsequent return to humanity, it is revealed that Muzan's soul was condemned to Avici, the lowest and most torturous level of hell, for the countless sins that he committed during his life. Here, Muzan is seen in his battle form and when asked about how it felt to be killed by a user of Sun Breathing, he declared that it was "very unpleasant."