Demon Slayers vs Muzan Kibutsuji is a battle that takes place in both Demon Slayer Corps Headquarters and Infinity Castle, pitting the Demon Lord Muzan Kibutsuji against a number of unnamed Demon Slayers, Tanjiro Kamado, Tamayo, and the Hashira.
Prologue
Muzan confronts Kagaya and Amane Ubuyashiki at the Ubuyashiki Estate. Muzan is dissatisfied with the Oyakata; he has lived for millennia without aging, whereas the Ubuyashiki family has been reduced to a leader who looks and smells like a corpse. Kagaya does not deny it, but says his goal is to defeat him with his entire life. He claims that he and he were once members of the same family, which he was unaware of. Muzan inquires as to what this means, and he responds that his family is cursed because one of their members, Musan, became a demon. Since then, all Ubuyashiki children have died at birth until they are told that they can redeem themselves and survive by dedicating themselves to the elimination of Muzan. Muzan speculates that Kagaya’s ramblings are the result of his illness, and claims that he is unpunished for killing thousands of people without suffering. Kagaya asks Muzan what he is dreaming about, and the demon, startled, responds that he dreams of being immortal, which he will be when he captures Nezuko.
Kagaya claims that only human thoughts are truly immortal, and that the murders he has committed in his lifetime have only served to fuel the rage of those who despise him, demon killers. Muzan is shocked when Kagaya tells him that his species will go extinct tonight. He yells at the Oyakata to stop talking, but it’s a contented Kagaya who says one final thing. He believes his Hashira does not love him enough, and that his death will only cause them to kill him more.
Muzan asks if he’s finished, and Kagaya thanks him for his time. At the same time, a Kasugai crow summons all Hashira to Kagaya’s domain, prompting everyone to flee to the mountain. The Demon Lord is bothered by his time on the mountain and wishes he could be back in his stronghold working on something else. He makes an internal remark about the strangeness of the Ubuyashikis, which enrages Kagaya. He tells Muzan that he graduated first in his class and that he is in charge of killing 300 demons as the leader of the missions against the Twelve Kizuki.
He warns the demon that the Hashira is on its way, and that he will be engulfed in a storm that will kill him, directly threatening and angering Muzan. Kagaya, Amane, their two current daughters, and Muzan’s body are all killed in a massive explosion that destroys the mountainside. When the Hashiras see the explosion, they are terrified and rush to the estate’s ruins. Muzan curses Kagaya for being mistaken, not believing that he would willingly kill himself and his family in order to kill him. He realizes that Demon Slayers must now be aware of his presence as he condemns him for hiding his bloodlust all along.
Battle
As Muzan tries to repair the damage, flesh seeds appear around him and burst into bramble branches, immobilizing him. He is suddenly attacked by Tamayo, who pierces his stomach with his hand and injects him with a drug, believing he can just absorb them. She informs him that he has been injected with a drug that she and Shinobu Kocho developed to transform demons into humans. Muzan is enraged by Tamayo’s intervention, reminding him that it was she who murdered his family, not him. Tamayo weeps as she expresses her desire to live and see her children survive, but she is also reminded that she killed people in order to survive. She then declares that they will work together to atone for these human deaths and asks for assistance.
Gyomei Himejima appears out of nowhere and beheads Muzan with his flail. Muzan, however, regenerates his head without difficulty, as predicted by Kagaya. Gyomei even claims that the sound of his regenerating flesh distinguishes him from any other demon he has ever encountered. He decides to keep him tied up until dawn, remembering his master’s words that sunlight is now the only option left. Muzan reflects Stone Breath, Third Form: Stone Skin with his Blood Demon Art: Black Blood, Stone Hashira Brambles. While the others arrive, worried about Kagaya, Sanemi sees the Demon Lord and screams, asking what he did to him. Gyomei informs the Hashira that the demon who has infiltrated their headquarters is Muzan Kibutsuji himself, and that no matter how many times he is beheaded, he will not die. This astounds all of Hashira, who has never seen Muzan in person. Tanjiro, the only person who has ever met him, reacts angrily when he sees him again.
To take down Muzan, the remaining Hashira use their respective breathing styles: Fourth Form: Shinobu Kocho with Insect Breathing, Butterfly Dance: Whim, Obanai Iguro with Serpent Breathing, Muichiro Tokito with Mist Breathing Mitsuri Kanroji and his Breath of Love, First Form: Winding Serpent Slash Swaying Love, Wildclaw, Giyu Tomioka with Water Breathing, Fifth Form Sanemi Shinazugawa with Breath of Wind, and Third Form: Flowing Dance Seventh Form: Strong winds with sudden gusts. Tanjiro Kamado enters the fray, wielding the Hinokami Kagura: Sunflower Push. All Demon Slayers converge on Muzan, who is immobilized. Instead of being concerned, Muzan smiles menacingly.
The aftermath of the fight
All Demon Slayers enter the Castle of Infinity through doors that open beneath them. Muzan taunts the Demon Slayers, telling them that they are now in hell and that he will slaughter them all tonight. Muzan mocks Tanjiro and challenges him to try if he can, to which Tanjiro responds by saying that it is Muzan who will get to Hell. Shinobu, cut off from the rest of the world, enters the castle and smells blood. She enters a large room filled with water and lilies, as well as wooden planks.
She sees someone inside who is surrounded by several female corpses. When the door creaks, the character turns to greet his new visitor. Tanjiro and Giyu collide, with Tanjiro nearly falling to his death before Giyu catches him. Tanjiro thanks the Hashira, but a demon appears behind him and kills him quickly with the Water Breathing, First Form: Slash from the water’s surface. Many more demons appear, forcing the two men to use the sixth form of Water Breathing, Whirlpool, and the third form of Water Breathing, Fluid Dance.
Tanjiro admires Giyu’s ability to use Breathing Styles and can tell what move he’s doing from the smallest movement he makes, allowing him to switch to another move without cutting himself. Obanai and Mitsuri collided, Obanai killing a slew of demons with the Serpent’s Breath, Fifth Form: Slithering Serpent. As he warns the demons not to approach Mitsuri, she extols Obanai’s strength and greatness, and he asks if she’s all right.
Sanemi, enraged at his inability to protect Kagaya, kills hordes of demons on his own and vows to slaughter them all. Gyomei and Muichiro work together to slay demons, to which Gyomei responds that each of the demons has the same power as a lower rank of the twelve Kizuki, believing that Muzan is sending them all at once to wear them down.Inosuke, Genya, and Zenitsu arrive at the Infinity Castle with their own agendas in addition to assisting Tanjiro and the Hashiras. Tamayo cries out for help, trapped inside Muzan’s cocoon of flesh.