Complex sentence: He makes his way back into the keep and finds Yumemaru with the woman who introduces herself as the Genma Hecuba as she assumes her true form while spiriting Yumemaru into the netherworld with Samanosuke in pursuit.
Split into simpler sentences: He makes his way back into the keep and finds Yumemaru with the woman, who introduces herself as the Genma Hecuba. She then assumes her true form and spirits Yumemaru into the netherworld with Samanosuke in pursuit.