After Kouga's defeat, Muramasa becomes unstable, as he had absorbed hollows in order to sustain himself without a master. Many hollows spill forth from his body, and he transforms into a form similar to an Arrancar. Orihime tries to reach out to him to save him from his sorrow, but he attacks the group. Ichigo manages to slash through Muramasa, but he becomes even more unstable and becomes a larger dome-shaped mass, engulfing him in the process. The formerly rebellious Zanpakuto return with their owners to help Ichigo and his friends in the battle. (thetvdb.com)