Ryujin

Ryujin sometimes known as the King of the Sea was an orochi king who entered into an alliance with the Ningyo race. In return for the Ningyo freeing the Orochi from Sakkaku, the realm of trickery, and allowing the Orochi to live in their cities the Orochi swore to protect the Ningyo eternally.

Ryujin noticed that ningyo appeared to be immortal. Ryujin himself was already immortal and so had no need or desire to betray his friends. Ryujin knew that when it was eventually discovered by other races that consuming ningyo flesh would give one immortality that the race would be hunted to extinction. Ryujin decided, then, to place a curse on the Ningyo to repel any who would dare slaughter his allies.

Major References

 * The Legend of White Shore Lake