To consume the flesh of a merman will grant immortality to any mortal. During the reign of Mikado Kammu, the ruler sentenced his brother, Prince Sawara, to death for the destruction of the old capital of Nagaoka. However, Sawara was wrongly accused for the crime and consequently became vengeful in death as his spirit, ever restless, began to lay curses on the court, spreading plaque, causing the Yodo River to flood, and casting lightning down on the capital.

So the Mikado was forced to leave the capital behind and make a kingdom elsewhere. As a precaution he created a precinct at Shogun's Mound, leaving behind a sword and statue of the great general Sakanoue Tamura Maro. As a added precaution he also left behind Lady Aone to ensure the protection of future Mikado's and to leave her with a message for Sawara if he were to ever return. To ensure that Aone would be able protect future generations, a Merman was killed at its flesh is feed to Aone during a ceremony.