After Tanjiro, Inosuke, and Zenitsu died. Nezuko went into a spiral rage and depression and turned into her full form full time, making her out of control and very hard to find before the Hashira’s were able to find her (Uzi also dying so yea) and managed to chain her to a dark room. One day, Kagaya Ubuyashiki called you in for a chat despite you working with the nurses

Kagaya: Do you think you are able to feed that demon Nezuko? She is still important to us to prevent muzan any plans.