Demon Slayers vs Tanjiro Kamado

Demon Slayers vs Tanjiro Kamado is the final battle between humanity and demons, taking place outside the destroyed Infinity Castle. It focuses on the battle between the last surviving remnants of the Demon Slayer Corps against the newly transformed Tanjiro Kamado.

Prologue
The Demon Slayers have managed to successfully expose Muzan Kibutsuji to the sun after a night-long battle until dawn. But the victory is pyrrhic; despite Muzan's death, nearly the entire Demon Slayer Corp is wiped out and the battle resulted in the eventual deaths of the Hashiras Gyomei Himejima, Obanai Iguro and Mitsuri Kanroji. The battle also led to the death of Tamayo and the remaining survivors all critically wounded.

As his armour of flesh burns away, Muzan sees Tanjiro Kamado inside, having been enveloped with him during his attempt to save himself from the sunlight. Tanjiro has been killed and his heart has stopped beating. Muzan remembers his past, having been close to death ever since he was born and his only dream was to conquer the sun as a demon to live forever. Recalling the words of Kagaya Ubuyashiki of human thoughts being the only true immortal thing, Muzan tears up and accepts he could never achieve immortality. But instead of accepting its true meaning, he decides his thoughts are immortal and instead of him, Tanjiro could be the one to carry out his will.

With the remains of his power, he pumps all his blood into Tanjiro's body, and as he himself begins to disintegrate, he tells Tanjiro that he will become the strongest demon of all, having Nezuko's immortality to sunlight and unable to be killed due to being the only one able to perform Sun Breathing. As the last portions of Muzan's body disappear, he voices that he believes that Tanjiro will not die and in his stead, he will kill the Demon Slayers.

Battle
Upon becoming a Demon, Tanjiro recovers from the poison that caused the growth on his face and regenerates his lost arm and eye. As Muzan predicted, Tanjiro shows resistance to sunlight when he faces off against Giyu Tomioka. He goes to attack Inosuke Hashibira, who saved Giyu, only to be embraced by a panicking and saddened Nezuko. Nezuko tearfully begs her brother to stop so they can return back home, and Tanjiro manages to temporarily calm himself down after he unconsciously bites Nezuko's shoulder. However, as Inosuke lands several punches to Tanjiro in frustration, Tanjiro further evolves his demon form and grows bone appendages similar to Muzan's.

Nezuko continues to beg to Tanjiro to not lose himself to Muzan but he continues to mindlessly attack the Demon Slayer Corps, swinging one of his bone appendages towards Zenitsu. Giyu saves Zenitsu and a few Kakushi using the Water Breathing, Fourth Form: Striking Tide. Tanjiro begins generating an energy ball to direct towards the Demon Slayers, however, his attack is intercepted by Nezuko and severely damages her arm. A powerless Zenitsu cries out to Tanjiro, horrified at the thought of Nezuko dying by the hands of her brother. As Giyu and an onlooking Yushiro try to find a way to stop Tanjiro, Kanao Tsuyuri arrives, bringing a vial of the medicine that could restore his humanity. Kanao uses the Flower Breathing, Final Form: Equinoctial Vermilion Eye on her left eye and approaches Tanjiro to inject him with the medicine, getting struck by one of Tanjiro's bone appendages in the process. In Tanjiro's mind, Muzan tries to convince him to let himself be consumed by the desire of becoming the ultimate being. Tanjiro vehemently refuses Muzan's offer and several hands belonging to his loved ones, both deceased and alive, rise from the ground and through the Wisteria flowers to push and reach out for him. Tanjiro accepts their grasps and is brought back to reality with his humanity restored. Muzan fades from Tanjiro's subconscious, finally defeated for good.

Aftermath
Tanjiro later awakens surrounded by a tearful Nezuko and the rest of the Demon Slayer Corps, gleefully welcoming him back. Kanao is later shown to be alive despite her injuries and she manages to share a heartful smile to Tanjiro, which he reciprocates. On the other hand, Yushiro, who is currently hiding from the sun, manages to find Tamayo's hairpin. He tearfully comments that the long-lasting battle between the Demon Slayer Corps and Muzan has come to a close.

Three months later, Kiriya announces that the Demon Slayer Corps will officially be disbanded and thanks the remaining active Hashira, Sanemi and Giyu, for their hard work. In turn, both Hashira thank the Ubuyashiki family for giving them the chance to avenge their loved ones, which greatly moves Kiriya. Meanwhile, Tanjiro is recovering in the Butterfly Estate, and comments that now Muzan's control over him is gone, his regenerated arm has heavily aged and lost its sensation from the elbow downwards while is regenerated eye is now blinded. However, Nezuko reassures him that this is for the best. After Tanjiro, Zenitsu and Inosuke are fully healed, they all move to Tanjiro's old house and live happily.

At some point before its disbandment, the Demon Slayer Corps members and their relatives took a commemorative photo together. The photo, along with Yoriichi Tsugikuni's Hanafuda earrings and Tanjiro's Nichirin Blade with Kyojuro Rengoku's hilt still exist in the present day, under the care of Kamado household.