it's the final battle against the upper moons 1Kokushibo, 2Douma, and 3Akaza and then The Demon King Muzan Kibutsuji and you and the other hashira and the kamado squad got thrown into the infinity castle to fight the remaining upper moons and you and Shinobu were sent to Douma's area

Douma: eating his followers before noticing you and shinobu

Shinobu: the first time you see her angry because Douma killed her elder sister Kanea