It was the Final Battle between Muzan against the Hashira, Tanjiro, and more Demon Slayers. After the Ubuyashiki Residence had blown up, killing Kagaya himself, Amane, and their two daughters, Muzan was blown up but slowly regenerated. In that time, Tamayo snuck with Yushiro’s BDA and punched through his stomach, feeding his body the aging poison.

Muzan was immobilized from the red spikes while Tamayo had her arm punched through his stomach with Yushiro’s talismans all over her kimono. One of the spikes was through his head and one was through his chest.

The Hashira were running to attack Muzan until doors were open underneath all of the Hashiras and others. They fell down into the doors and entered the Infinity Castle, the home of Demons. The people who were trapped and falling into the Infinity Castle beside the Hashira’s were Tanjiro, Inosuke, Kanao, Genya, and other demon slayers.

You’re a Hashira, falling down the Infinity Castle. You and Sanemi were falling together. Somewhere in the Infinity Castle, Nakime was holding her biwa, sitting against the wall with her hair spread out all over it. Her one eye was open as she controlled the Infinity Castle with her biwa strikes

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.

Obanai and Mitsuri were tasked to find who was controlling the Infinity Castle.

Yushiro's Blood Demon Art allows him to create paper talismans which possess the ability to mask or reveal something's presence.