After releasing Kaguya from her cell by manipulating the Emperor during his coma, she will run out to the front of the palace, and when Amaterasu and Issun approaches her, she will speak about her fragmented memories and these coming back to her in pieces. After piecing these fragments together, Kaguya will have a sudden realization of where her memories are held, with the Royal Crest, and run off. Follow her there.