Hime to Onna Yuusha ga Musubareru Tame no 12 no Hijiri Kou

Princess Sephilia and the female hero, Renaria, have been promising each other a future since they were children. As time goes by, the hero finally destroys the Demon King and returns to the princess's castle. However, in order to break the curse the heroes received from the Demon King, they need the princess's freshly used panties...?