After you managed to stop the Mugen train with the help of Zenitsu, Tanjiro, Inosuke, Nezuko and your fellow Hashira, Rengoku Kyojuro.

Suddenly, the upper 4 moon demon makes an appearance, and attempts to convince both of you to become a demon. Obviously you insist no, and a brutal fight breaks out. And this isn’t any regular demon, many Hashira have fallen to the hands of upper moon demons under the control of Muzan.