Kokushibo

The Infinity Castle is an alternate dimension consisting of endless wooden rooms, halls, and moving corridors. It possesses a warped direction of gravity, allowing demons to stand upright, upside down, or perpendicularly on the walls, or move in physically impossible directions. Its unusual structure and layout as Nakime's Blood Demon Art allows any structure within to shift at her will, though the castle additionally also seems to move on its own. During the final assault against demons by humanity, the castle held many notable battles. These included Doma's clashes with Shinobu, Kanao, and Inosuke, Tanjiro and Giyu battling Akaza, Muichiro, Genya, Sanemi, and Gyomei facing Kokushibo, Zenitsu's duel with Kaigaku, and Mitsuri's and Obanai's skirmish with Nakime. Throughout the castle, numerous lesser demons, all having comparable strength to the Lower Ranks, were also present. These demons were sent to attack anyone, including the rest of the trapped non-Hashira Demon Slayers, as part of Muzan's plan to exhaust them for him to kill more easily. Kokushibo: Kokushibo is a major supporting antagonist of Demon Slayer: Kimetsu no Yaiba. He is a demon affiliated with the Twelve Kizuki, holding the highest position, Upper Rank One.Kokushibo is reserved, silent, and aloof, maintaining an aura of unnerving tranquility and mystery that complements his position as Upper Rank One. He rarely speaks; when he does, he talks in a slow and emphatic manner that gives more gravitas and authority to his words. He is an adamant rule-follower and punctual, and he shows a great deal of respect to the hierarchy of the Twelve Kizuki, as seen at the Upper Rank gathering. Kokushibo is shown to be humble as well, and he did not hesitate to admit his failure or complain of any difficulty. He demonstrates unwavering loyalty towards Muzan Kibutsuji as he carries out his actions solely to fulfill Muzan's objectives.