The Yata-no-Kagami is an artifact—an ancient mirror which is one of the three Japanese imperial regalia, along with the Kusanagi Sword and the Yasakani-no-Magatama. In Mutant Ninja Turtles Gaiden, it is also a supernatural object and important plot device, whose nature is explained in Chapter 21:The Curse of "Yamata-no-Orochi". Spoiler warning: Plot or ending details follow.