Demon Slayers vs Muzan Kibutsuji is a battle that takes place in both the Demon Slayer Corps Headquarters and the Infinity Castle which occurred between a number of unnamed Demon Slayers, Tanjiro Kamado, Tamayo, and the Hashira against the Demon Lord Muzan Kibutsuji.
As Muzan continues to regenerate, he sees multiple pieces of burning flesh around him. These pieces of flesh are in fact Tamayo's Blood Demon Art, Flesh Seeds. The flesh seeds then burst into black flesh trees with spikes puncturing his flesh so he can't pull them out. As Muzan attempts to absorb the Blood Demon Art, Tamayo rushes in and jams her fist into his abdomen. She then questions him what she held in her fist that Muzan absorbed, a special drug that reverts Demons back to humans. Muzan thought that it was impossible for demons to become humans again but Tamayo has finally solved it with help from Shinobu Kocho. Muzan then tells Tamayo that she is very stubborn and that her hatred towards him is unjust, due to her killing her family herself and not Muzan. She responds that due to Muzan turning her into a demon and that her only intent on becoming a demon was to survive her illness and see her children grow up. Because of this, she will atone for her sins and will sacrifice herself so both Muzan and her will die, ending the demon race forever.
The Drug to end Demons
Suddenly, Gyomei Himejima appears and crushes Muzan's head with his spiked flail. However, with Kagaya Ubuyashiki's prediction, Muzan regenerated his head with little to no difficulty. Gyomei even stated that due to the sound of his flesh regenerating, he is unlike any other demon he has ever faced. Thus Gyomei knows that he, Tamayo and the Demon Slayers have to restrain Muzan until the sun rises. Muzan uses his Blood Demon Art, Black Blood: Brambles at the Stone Hashira in which he reflects it with the Stone Breathing, Third Form: Stone Skin. Sanemi sees the Demon Lord and screams, asking what he did to Kagaya with the other Hashira worried sick as well. Gyomei screams at the Hashira that the demon who infiltrated their headquarters is none other than Muzan Kibutsuji himself and that he will not die no matter how many times he's beheaded. This greatly shocks all the Hashira beyond measure due to them never seeing Muzan in the flesh, however with Tanjiro, instead of being shocked he's furious with seeing Muzan's face once again.
The Hashira Appear
All the remaining Hashira use their respective Breathing Styles in order to cut Muzan down: Muichiro Tokito with the Mist Breathing, Fourth Form: Shifting Flow Slash, Shinobu Kocho with the Insect Breathing, Butterfly Dance: Caprice, Obanai Iguro with the Serpent Breathing, First Form: Winding Serpent Slash, Mitsuri Kanroji with her Love Breathing, Fifth Form: Swaying Love, Wildclaw, Giyu Tomioka with the Water Breathing, Third Form: Flowing Dance, and Sanemi Shinazugawa with the Wind Breathing, Seventh Form: Gale, Sudden Gusts. Tanjiro Kamado joins in the fight and uses the Hinokami Kagura, Sunflower Thrust. Thus a huge barrage of Demon Slayers surround Muzan. However, instead of being concerned, Muzan gives off a disturbing smile.
Tanjiro and the rest of the Hashira, including many Demon Slayers are teleported inside the Infinity Castle separated through many openings. Muzan mocks the Demon Slayers and asks if they really thought that they cornered him. He then violently screams that they are in hell now and that he will slaughter them all tonight. Tanjiro refutes and says the it is Muzan who will arrive in hell, to which he scoffs and dares Tanjiro to try if he can.
Shinobu Kocho all alone smells the scent of blood keeps walking inside the Infinity Castle. She walks into a large water filled room with lilies and wooden planks, inside she sees a large male figure with multiple female corpses all around him. The figure hears the door creak and turns around with him eating a disembodied arm. She sees the kanji for Upper Rank (
Tanjiro Kamado and Giyu Tomioka were separated together, Tanjiro nearly fell to his death if not for Giyu catching him. Tanjiro thanks the Hashira with a demon creeping up behind him. Tanjiro however easily kills the demon with the Water Breathing, First Form: Water Surface Slash. Multiple other demons follow requiring both to use the Water Breathing, Sixth Form: Whirlpool and Water Breathing, Third Form: Flowing Dance. Tanjiro venerates Giyu for his skills on using Breathing Styles and can tell which move he does just from the slightest movement he did so he may also use another move without cutting each other.
Obanai Iguro and Mitsuri Kanroji were separated together, with Obanai using the Serpent Breathing, Fifth Form: Slithering Serpent to kill a plethora of demons. While he threatens the demons to stay away from Mitsuri, she gushes over how strong and great Obanai is with him asking her if she is okay. Gyomei Himejima and Muichiro Tokito are together killing demons, in which Gyomei stated that each of the demons have the same power of a Lower Rank of the Twelve Kizuki.
After the fight between Muichiro Tokito, Genya Shinazugawa, Sanemi Shinazugawa, Gyomei Himejima and Upper Rank One Kokushibo, an enormous capsule of flesh is revealed with multiple appendages connected to the walls of the Infinity Castle with Tamayo stuck on the surface of it. A large group of normal Demon Slayers surround the capsule of flesh and attempt to cut all the appendages off.