Tanjiro Kamado vs Demon Slayers is the final battle against Demons in the Kimetsu no Yaiba universe, taking place outside the destroyed Infinity Castle. It focuses on the battle between the newly demonized Tanjiro Kamado against the surviving members of the Demon Slayer Corps.
As Muzan Kibutsuji lays dying, he transforms Tanjiro Kamado into a Demon and transfers all of his memories to the young Demon Slayer in a desperate effort to preserve himself. Muzan entrusts Tanjiro with wiping out the Demon Slayers and believes that Tanjiro would, like his sister, conquer the sun.
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 had predicted, Tanjiro shows resistance to sunlight when he faces off against Giyu Tomioka. He goes to attack Inosuke Hashibira, only to be embraced by a panicking and saddened Nezuko. Nezuko tearfully begs her brother to stop so they could 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.
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 humanification medicine.
Kanao uses Flower Breathing: Final Form - Equinoctial Vermilion Eye to approach Tanjiro and inject him with the medicine, getting struck by one of Tanjiro bone appendages in the process. In Tanjiro's consciousness, 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 is left alone in Tanjiro subconsciousness, finally defeated for good.
Tanjiro later awakens as he's surrounded by a tearful Nezuko and the rest of the Demon Slayer corps. They all welcomed his comeback gleefully. Kanao is later shown to be alive despite her injuries and she manages to share a heartful smile to Tanjiro, which he reciprocates as well. On the other hand, Yushiro, who currently hiding from the sun manages to found Tamayo hairpin and commented that 'It's all over', thus finally put the long-lasting battle between Demon Slayer Corps and Muzan to a close.
Three months later, Kiriya announces that the Demon Slayer Corps will be officially disbanded and thanked the remaining active Hashira, Sanemi and Giyu for their hard work. Both Hashira instead thanking the Ubuyashiki family since they had given them the chance to avenge their loved ones, which greatly moves Kiriya. Meanwhile, Tanjiro is recovering on the Butterfly estate, and commented that now Muzan controls over him are gone, his regenerated arm heavily ages and lost its sensation from the elbow downwards. His regenerated eye is also blinded. However, Nezuko reassures him that this is for the best. After Tanjiro, Zenitsu and Inosuke are fully healed, they all moved to Tanjiro's old house and lived happily.
At one point before it's disbandment, the Demon Slayer Corps members and their relatives took a commemorative photo together. The photo, along with Yoriichi's Hanafuda earrings and Tanjiro Nichirin blade still exist in the present day, under the care of Kamado household.