Muzan Kibutsuji/Synopsis

Past
When Muzan was in his mother's womb, his heart stopped multiple times. He was said to have been stillborn, having no pulse and not breathing. However, when he was taken to be cremated, he released his first cries.

As a young human man, Muzan was diagnosed with a disease that would kill him before he reached the age of 20. The doctor treating Muzan gave him a special prototype medicine. In a fit of rage, resulting from his worsening condition, Muzan killed the doctor. However, only after killing the doctor did he realize that the medicine was actually working. He had acquired a strong body. Muzan had no problems adjusting to having to kill other humans for survival, but the problem he faced was the fact that he was unable to walk in the sun. The sun was fatal to him, forcing him to resort to doing his activities only during the night, a condition that he resented. It was at that moment, that he truly regretted killing the doctor, before he could have finished his treatment with his "Blue Spider Lily" medicine. He searched the entire country for the Spider Lily, but couldn't find it. Due to this, Muzan started to create more Demons. Planning to spread them in the world, hoping that one of them would eventually find a way to overcome the sun or at least help him on his quest to find the Blue Spider Lily. He then wants to use this information to turn himself into a truly perfect being.

Around 400 years before the main story, Muzan found Tamayo, the latter begging the former for a way to cure her of her illness so she could live with her family. Muzan turned her into a Demon who went berserk and massacred her family. Muzan taunted her for what she did while Tamayo was horrified. From that point onwards Muzan kept Tamayo close to himself, even tho she deeply resented him.

During this time he also found Michikatsu Tsugikuni a Demon Slayer and user of the Breath of the Moon style. Knowing Michikatsu feared his life would end at 25 years old, the Demon Lord tempted him with promises of immortality and power. Michikatsu who was always jealous of his younger brother accepted the offer and became the Demon Kokushibo and from that point onwards, Muzan's central lieutenant.

Soon after he along with Tamayo encountered Yoriichi Tsugikuni, a Breath of the Sun user, Muzan told him that he lost interest in swordsmen who could use breathing techniques seemingly hinting at how he recruited his brother. Muzan displayed a boiling violent drive that terrified Yoriichi, as the Demon Lord attacked Yoriichi he could sense that any of his attacks would be fatal and felt fear for the first time in his life. He used the opportunity to perfect his techniques and realized Muzan's abnormal anatomy of 7 hearts and 5 brains powering him. Unleashing his 13 techniques in succession, Yoriichi was able to destroy most of Muzan's vital organs leaving him crippled and struggling to keep his head attached. Yoriichi noticed Tamayo who was hopeful his hated foe would perish. Yoriichi asked Muzan what was life worth to him, but he did not respond, turned red seemingly from rage and erupted in 1,800 lumps of flesh. Although Yoriichi was able to cut 1,500 of them, he correctly deduced the other 300 most have reformed into a head to regenerate. Ever since then, he has been more vigilant and careful about encountering possible Breath of the Sun users, who can be identified by a pair of Hanafuda earrings they usually wear, which also made the Kamado family a target, as they practice the Dance of the Fire God, which is the most closely-related breath to the Breath of the Sun. To that end, he, along with Kokushibo were hunting people that possibly possess Hanafuda earrings.

Hundreds of years later and into the present, Muzan was able to locate the Kamado family who were keepers of the Breath of the Sun technique. He stormed into their house and proceeded to inject them with his blood hopeful that he could create a Demon who could withstand the sun. He grew disappointed when the family died from his blood's poison, believing that creating a sunproof Demon was not so easy after all. He left their home seemingly unaware that there was a survivor in Nezuko Kamado who became a Demon.

First Mission Arc
He is first ever mentioned when Sakonji Urokodaki tells Tanjiro Kamado that he is the enemy of the Kamado family and that he might know how to turn Nezuko Kamado back into a human.

Asakusa Arc
Traveling through a crowd in Asakusa with his family, Muzan is accosted by an angry Tanjiro. Muzan's daughter asked him who the young Demon Slayer was, and he asked the youth if there was something he wanted from him. His wife then approached them, asking if Tanjiro was somebody familiar to their family. Muzan denied this, suggesting the youth had mistaken them for somebody else. He then quickly slashed a passing man, turning him into a Demon. During the commotion, he and his family leave while he comforts them.

Afterwards, Muzan sends his family home in a carriage without him, declaring he has some work to do. He wanders through an alley, where a drunk man bumps into him. The man then mocks and insults him, saying he doesn't like wealthy people like him. When the drunk man notes how pale his skin is, Muzan smashes him into a wall and also proceeds to kill the man's older brother. He then turns to the drunk man's cowering girlfriend and asks if he looks pale or close to death. Muzan answers his own questions and tells her how he is infinitely close to perfection. He then drills his finger into her forehead, pumping her with so much blood that her cells melt. Finally, he calls Susamaru and Yahaba and orders them to kill Tanjiro.

Rehabilitation Training Arc
Since Muzan is able to discern demons' location whom him has given blood and thanks to Nezuko being taken into Ubuyashiki's headquarters, now he knows where to find him out.

The Lower Moons have a meeting, where Muzan appears in the form of a woman. He is very disappointed after Rui's defeat, so much so that he declares that he'll disband the Lower Moons and that he will now lead only the Upper Moons. Then he starts killing everyone. Rokuro still pleaded with him and asked for more of his blood to prove himself. Since Muzan sees him as a failure, this statement makes him incredibly angry and he killed him as a resulf of being contradicted on the way he responded to that request. Before killing Enmu, he let him say his last words. When Enmu states how much he enjoys making humans suffer, Muzan sees some potential and gives him a lot of blood. Enmu almost dies from the new load of power, but manages to survive. Muzan then gives him one last chance to prove that the Lower Moons aren't that useless.

Demon Train Arc
Muzan also leads another life among humans as a small boy. He pretends to have a skin condition, due to which he can't step into the sun. After killing Kyojuro Rengoku, Akaza reports to Muzan and tells him of his victory. But he isn't so pleased by the death of the Hashira, instead showing anger that Akaza let all the other present Demon Slayers get away and that Akaza has yet not found any hints for the Blue Spider Lily.

Entertainment District Arc
In the Red Light District, Muzan talks to Daki. He tells her how proud of her he is and that he hopes she will become even crueler and stronger. He also lets her know about Nezuko and leaves it to her to kill her. After the fight, he orders the Upper Moons to meet in the Dimensional Infinity Fortress. He is standing on the ceiling and experimenting, trying to heal his sun weakness. Muzan tells them that he won't allow any failures such as Daki and Gyutaro. Muzan regrets putting so much confidence in her and wishes Gyutaro had fought since the beginning. He tells them to serve him with more suicidal devotion and states how he hates change. He believes change only means decline and he didn't change for so long, since he is practically perfect. He then sent Gyokko and Hantengu to destroy the swordsmith village before departing from the meeting.

Swordsmith Village Arc
Muzan feels through Hantengu that Nezuko was indeed able to become a Demon who overcame the sun, thus making his assault on the Breath of the Sun family a success. He was in the disguise of a small boy when he hears of this. His foster mom and maid come in but Muzan instantly kills them. He is extremely excited and sets his mind on getting Nezuko.

Hashira Training Arc
He appoints Nakime as Upper Moon Four and makes her more powerful. He uses her to scout out the whereabouts of all Demon Slayers. He is quite impressed by her efforts and compliments her. He says that he is sure to soon find Nezuko and Kagaya.

Dimensional Infinity Fortress Arc
Muzan appears at the Ubuyashiki mansion and sees Kagaya. He mocks him for his weak appearance. Kagaya tries to lecture Muzan and points out how the Hashira will never give up trying to kill him. Muzan laughs and proceeds to attack Kagaya. Before he can kill him however, the mansion explodes, with Muzan caught in it. The explosion, which was enhanced by spike traps, manages to injure Muzan. Before he is able to regenerate fully, Tamayo appears and uses her Blood Demon Art to keep him in place, while using a drug that can turn him into human. Using this opportunity, Gyomei Himejima appears and destroys Muzan's head. However he is able to instantly regrow his head. As all of the other Hashira join the fight, the Dimensional Infinity Fortress appears as if sucking in all the people nearby and scattering them. Muzan claims he will slaughter them all tonight. As the Demon Slayers manage to beat the majority of the Upper Moons and attempt to locate Muzan, several lower ranked members are able to find his cocoon. Emerging from it, he proceeds to eviscerate all the Demon Slayers in his proximity, as he tries to regain the stamina lost from his struggle against Tamayo. He takes her head and mocks her for being unable to kill him, ridiculing that the drug she designed was useless. Tamayo angrily demands Muzan to return her family to her, to which Muzan replies she should join them on the other side and crushes her head, which Yushiro senses. He further proceeds to slaughter Demon Slayers and reaffirms his vow to kill all of them.

Through Nakime's ability, Muzan is placed face to face with Tanjiro and Giyu. Muzan asks Tanjiro why he is so concerned with revenge against him, when he got out of the massacre of his family alive, and thus can resume his life if he wanted to. Comparing himself to a "Great Calamity" he states that it is impossible for humans to take revenge on natural disasters like rain, winds, volcanic eruptions or earthquakes take the lives of dear ones, humans keep on living, and that fighting him amounts to the same kind of folly. He thus concludes that the Demon Slayers fight him because they are a group of deviants, and that he has tired of dealing with them and is the only one who wants to truly put and it to the fight. An enraged Tanjiro claims Muzan should not have been allowed to exist.

A battle ensues with Muzan extending his arms into long bladed whips keeping Giyu and Tanjiro at bay, as the latter infuriated at Muzan's inhumanity Tanjiro navigates between Muzan's strikes and attempts at infiltrating but is ultimately unsuccessful and slashed near his eye. Giyu pulls Tanjiro away reminding the latter that Muzan's power is above any of the Upper Demons. Muzan wonders if they can defeat him with just three Hashira, as he declares that Mitsuri and Obanai were killed by Nakime. Muzan performs more sweeps that Tanjiro can hardly avoid due to his injured eye, as he stumbles. Before he can kill Tanjiro, Mitsuri appears smashing the walls and attacking Muzan with her Breath of Love: Kitty Paw Love Breeze, but is unable to inflict any damage on Muzan, much to her frustration. Obanai also appears revealing both Hashira to be alive.

An angered Muzan demands explanations from Nakime only to learn that Yushiro has used his own Blood Demon Art to take control of Nakime and that he will force Muzan out of the Infinity Fortress. Further angered by the opposition, Muzan tries to wrestle control of the Fortress back from Yushiro as he tries to absorb his cells through Nakime, an attack from Giyu and Obanai, force Muzan in the defensive allowing Yushiro to win control, so Muzan remotely detonates Nakime crushing her head. While her remaining cells still alive, Yushiro uses them to take control of the Fortress as it continuously shifts. Muzan performs more sweeps which manage to slash Tanjiro, as the Fortress shakes being forced into the outside. Before Muzan can land a critical blow on Mitsuri, Tanjiro throws a broken Nichirin Blade which stabs Muzan in his head, causing him to miss. He grows furious with Tanjiro as the Fortress breaks into the surface.

Sunrise Countdown Arc
Now at a city, the Demon Slayers are told to stall Muzan for one more hour and a half until sunrise. A furious Muzan now with whips from his spine, emerges from debris and dares them to try, wildly swinging his limbs. The Hashira seize a chance to slash him, but to their surprise he regenerates so fast that he cannot be decapitated. Before he can fatally wound them, hordes of lesser ranked Demon Slayers push the Hashira out of harm's way, ready to die as human shields as Muzan murders dozens of them. He muses that they were fortunate to die immediately and points to Tanjiro as an example of the contrary. Showing a horrible facial burn, Muzan reveals he injected his blood into Tanjiro on his earlier attack, and that his body will be destroyed, boasting that he is already dead.

Muzan and the Hashira exchange blows and notices that Demon Slayers with their mark activated can resist the effect of his demonic blood. As Muzan tries to land a killing blow on Mitsuri, Gyomei saves her. Sanemi takes the chance to slash Muzan from behind who reacts to the Hashira's attack. Sanemi tosses vials filled with flammable liquid and proceeds to set Muzan ablaze, to which the progenitor Demon protests as dirty. Sanemi proceeds to insult him and say he is the dirty one and vows to kill Muzan.

As the battle rages on the Hashira have difficulty in fighting Muzan who proves to be too powerful for them. An exhausted Mitsuri who does nothing but evade is ultimately injured and forced out of the fight despite her desire to battle. Obanai takes notice and tries to fight harder against Muzan, trying to slice and kick his severed member away to delay his regeneration, but only making a minimal difference. Remembering his own past, decides to fight for her sake and die to meet her in another life.

Muzan's blood begins to damage the Hashira, Giyu loses the ability to hold his Nichirin Blade, with only a little more than 15 minutes passing. When all seemed lost, Chachamaru, Tamayo's cat servant, appears and throws syringes at the Hashira, dosing them with a serum that counteracts Muzan's poison. Muzan kills the cat, and grows annoyed how Tamayo still gets in his way even after death. Obanai thinks in how Muichiro may have managed to get his blade to turn red, and puts all his focus and effort into his grip, turning it red, however, he starts to lose consciousness due to lack of oxygen, barely able to avoid Muzan's attacks and Giyu unable to reach in time. Obanai is sent mid-air and miraculously manages to avoid Muzan's attack as Muzan's right arm and spinal whips are sliced. Muzan is left wondering how did it happen and realizes it is impossible for any of the Hashira to have been able to cut him, he is then sliced in his left arm now which makes him notice the uneven cut he has in his arm. Muzan correctly deduces that somebody is attacking him while hidden through Yushiro's technique. Feeling the air around him parting, he narrows the location of his three attackers, being none other than Inosuke, Zenitsu and Kanao whom he manages to barely slash. Gyomei is surprised to see them alive and Inosuke boasts that he has an enormous supply of Yushiro's paper so whatever Muzan tries is useless but is scolded for chatting more than necessary.

Obanai takes the chance to slice Muzan's arms which regenerate slower due to his red blade, a fact Sanemi points out. Zenitsu and Kanao turn invisible again and attack Muzan in unison, causing the demon lord to complain in irritation at their small tricks. Gyomei is glad to have more support as he is able to clash his axe and iron ball to heat them through friction, increasing their attack power. Gyomei soon smashes the left side of Muzan's torso. Sanemi and Giyu follow suit and clash their blades turning them red. The Kasugai Crow warns that 1 hour and 3 minutes remain before dawn, with the Demon Slayers regaining advantage.

The Demon Slayers take advantage of Yushiro's papers allowing them to see one another and launch a relentless assault on Muzan. Gyomei concentrates and is able to discern Muzan's unnatural biology, advising them to concentrate so they are able to see them too. Suddenly Muzan performs an undisclosed technique allowing him to defeat all his attackers simultaneously, injuring and having them crash on nearby buildings, mutilating Gyomei's left leg and Giyu's right arm in the process. Kanao Tsuyuri is left with a broken blade kneeling in shock. Before Muzan can kill her, a recovered Tanjiro saves her slicing Muzan's arm leaving him shocked, and has her taken to a medic Demon Slayer for safety. Seeing Tanjiro's mutated face as a result of his poison, Muzan comments that he looks terrible, and wonders now who is the true Demon between the two of them. Seeing Yoriichi's image in Tanjiro, Muzan feels disgust at the Demon Slayer, while Tanjiro states it is time they finish things.

Their battle begins with Tanjiro remembering the forms of the Breath of the Sun he witnessed during his dream, and his father's advice, he comes to realize that the forms must be employed consecutively one after the other, which will lead to the 13th and strongest form. Though he feels discouraged at not having Yoriichi's or his father's skill Tanjiro nonetheless ventures to attack Muzan who sees the ghost of his former enemy on the younger Demon Slayer. Tanjiro is able to discern Muzan's secret technique, an array of additional whips catapulted from his tights at immense speed taking his enemies by surprise after accustoming them to his current attack style. Turning his blade red, Tanjiro begins to unleash the Breath of the Sun forms, trying to become stronger in the brink of death.

Muzan is unimpressed as Tanjiro's technique is far from Yoriichi's level, seeing him losing strength he tries to secure the chance to end Tanjiro once and for all who is weakening from exhaustion and his own injuries. Muzan, however is unable to land a finishing blow much to his own surprise and realizes he too is growing weaker. He deduces that it must be Tamayo's doing as he had absorbed her cells and tries to communicate with her essence in the cells to ask what she the drug she injected him with was truly about. Tamayo's essence merely mocks him, much to Muzan's irritation who proceeds to destroy what remains of her consciousness and scan her memories instead. Witnessing her memories he comes to learn the truth, that Tamayo, Yushiro and Shinobu Kocho did indeed develop a drug that turned Demons into humans but added an additional poison with a powerful aging effect that would trigger with greater potency on its own in the case the humanizing venom would fail, making Muzan age at a rate of 50 years per minute. Muzan realizes that his body has weakened as it is struggling to keep his body youthful. He begins calculating the time the drug has been active, for roughly 3 hours, meaning he has aged 9,000 years. Tanjiro keeps unleashing successive forms of the Breath of the Sun irritating the Demon Lord with his persistence. With 59 minutes left before dawn Tanjiro is able to connect all the twelve forms and resolves himself to reuse them all faster than before.

Chachamaru is revealed to have survived Muzan's attack as Yushiro had demonified the cat before the fight. He proceeds to nurse the injured Demon Slayers. He concludes that Gyomei is unable to fight any longer due to blood and limb loss, before treating the others.

Muzan continues to lash at Tanjiro who is slowly being overcome with oxygen deprivation and losing his footing, but is saved by Obanai who was restored to health by Yushiro and who is revealed to have lost his sight during Muzan's earlier use of his leg tentacles. Tanjiro is worried but Obanai reassures him he can see through Kaburamaru. The two split as Muzan chases after Obanai and obliterates a building trying to kill him but fails. Muzan is in disbelief that a mere snake can help the Demon Slayer read his attacks, as Obanai and Tanjiro combine attacks. As Muzan grows weaker from the aging venom and the efforts of the Demon Slayers, scars become visible across all of Muzan's body. Tanjiro realizes those are the wounds inflicted by Yoriichi, burned down to a celular level which never healed despite hundreds of years and which reveal his weak points.

Muzan grows horrified at the sight of his wounds, thinking that Yoriichi, a mere man who displayed no apparent hostility was in fact the true monster between the two of them by crippling him the way he did. Tanjiro and Obanai continue their assault on Muzan now with 40 minutes left before dawn. Muzan grows restless and tries to escape trampling over the corpses of the Demon Slayers he slaughtered. This enrages Tanjiro who proceeds to throw multiple Nichirin Blades at Muzan who deflects them reasoning that stabs are much worse than mere slashes now. Obanai stabs Muzan in the neck from behind, making the latter use his spinal whips which are stopped by Tanjiro, who tries to give Obanai a pair of Yushiro's blindfolds so he can share Kaburamaru's vision. Although Muzan wounds the two of them to block their efforts, Kaburamaru succeeds in fetching the papers allowing Obanai to fight in much better condition. Muzan slowly succumbs into panic as he is running out of breath as his strength is reaching critical lows. Their battle is witnessed from afar by Nezuko who cries out of her right eye.

Muzan is further pressured by the attacks of Tanjiro and Obanai and with 35 minutes before dawn, Muzan deems it an unnecessary risk to continue the fight and attempts to divide himself into lumps, escaping in the same manner he did against Yoriichi. Tanjiro alerts Obanai of this and panics at the idea that the two of them won't be able to destroy all the flesh lumps. However, Muzan is unable to split as a 3rd drug injected by Tamayo has the property to slow down cell division. Muzan communicates with the essence of Tamayo who gleefully alerts him that there were not three effects but four, with the fourth drug causing Muzan to cough blood as it triggers cell destruction after the 3rd effect activated. The essence of Tamayo taunts Muzan that the death he so much feared is coming for him, further elaborating she doesn't need to be stronger than him to kill him as weakening him suffices, taunting him that just as he will do everything to live, they will do everything to kill him.

Muzan ponders that normally the attacks used by the Demon Slayers shouldn't be threatening nor should prolonged regeneration prove tiring, but Tamayo's drugs have turned otherwise manageable attacks into lethal threats and that he is unable to fully get accustomed to the toxins in his body which are much different than those used to kill Doma. In a desperate attempt to live, Muzan opens a gaping mouth in his chest and unleashes a potent discharge against Obanai and Tanjiro, leaving the latter convulsing and stunned. With just 25 minutes left before dawn, the weakened Kiriya who witnesses the fight from afar begs for someone to stall Muzan. His pleas are answered when Inosuke attacks Muzan lunging at the Demon Lord furious for the previous thrashing and mourning the comrades Muzan has killed and injured. Muzan unable to further use his previous technique retorts to lunging his arm into the ground wrapping Inosuke with his tentacle.

Inosuke is saved by the timely intervention of Zenitsu who tells Inosuke that Tanjiro is still alive, begging the latter to remain alive for the sake of Nezuko who is returning to normalcy. Muzan swings his arm at Zenitsu crashing him into a building, but is saved by Inosuke who stabs his arm only to be swathed away by another swing in the process. Muzan is under heavy fatigue and irritated as the boys continue to stand up to fight him.

Inosuke and Zenitsu continue his assault on Muzan with the latter using his Seventh Style: Flaming Thunder God managing to slash Muzan but the attack was more shallow than expected. Inosuke keeps attacking Muzan with his techniques forcing the Demon Lord to employ its shockwave once more. Before he can attack Inosuke, Tanjiro recovers and unleashes successive Breath of the Sun techniques realizing Muzan needs to recharge before using his shockwave attack. Tanjiro weakens after using Dragon Sun Halo Head Dance, giving Muzan an opening, but has his tentacles sliced by Inosuke. Zenitsu tries to join the fight but is unable to muster enough speed to use his techniques. Muzan swathes Inosuke away to a building and Tanjiro loses footing, but Zenitsu manages to recover enough to use one last attack, but is himself injured by Muzan. Tanjiro seizes the chance and stabs Muzan pinning him to a building, unable to use any more techniques as he gambles it all in keeping Muzan in place. Muzan tries to kill Tanjiro but a recovered Kanroji entagles herself to his tentacles while ripping his left arm tearfully yelling him to stop, while Muzan's right arm and tentacles are sliced by Sanemi. Left with little choice, a bifurcation appears in Muzan's front, revealing an enormous bestial mouth trying to eat Tanjiro who cannot do anything but remain in place. To protect him, Obanai puts himself in harm's way as he is mauled by Muzan, as dawn is breaking.

Muzan panics at the sight of the sun and desperately unleashes a powerful shockwave, catapulting his attackers away except for Tanjiro who loses his left arm but firmly stays in place trying to turn his blade red, when Giyuu appears to aid him and together the blade changes injuring Muzan. In response to the sun's menace Muzan proceeds to enlargen himself adopting a large demonic baby form absorbing Tanjiro in the process. As Muzan tries to get into shade, hordes of Demon Slayers block his path by tossing bookshelves and ramming vehicles on him, as he fights back for a way through. He is soon blocked by another group of Demon Slayers pushing a bus on him to prevent his escape. Muzan soon begins burning from the sun forcing him to fight for his live. Before he can overwhelm the lower ranked Slayers, Sanemi appears and slices his right arm, cursing the Demon Lord's persistence and demanding him to die.

As he tries to climb over the bus blockade, he has a chain tied to his neck by Gyomei, aided by multiple Slayers who pull together to bring Muzan down. As he burns further, Muzan resorts to digging as a escape maneuver forcing Giyuu, Sanemi and Obanai to attack him to deplete his stamina. Soon the chains breaks in what appears to be a hopeless moment, but Tanjiro who remains within Muzan is able to ignite his blade and injure him from the inside causing Muzan to flinch from the pain as he begins disintegrating from the sun's light. As his body fully burns into ash, the once invincible Demon Lord who ruled the shadows of Japan, spreading misery and suffering for a millenia, finally laid in defeat, ending the seemingly eternal war with the Demon Slayers' victory.

Legacy
Muzan's defeat while significant, would prove to be a costly victory for the Demon Slayers, leaving scores of members deceased, dozens mauled and heavily injured and many on the verge of death.

However, despite Muzan's death, with his last breaths, he managed to turn Kamado Tanjiro into a demon. As Muzan transfers his blood and memories into Tanjiro's dying body, he is able to successfully revive him as a Demon, deeming him the King of Demons he influences Tanjiro into conquering the sunlight and achieve his dream to kill the Demon Slayers.

Tanjiro's poisoned eye is restored along with regenerating his left arm, as the young Slayer rampages in a feral state against the rest of the Demon Slayers and successfully overcoming the sun.

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Giyuu Tomioka Гию Томиока 富岡義勇