What is "yofune-nushi"

Yofune-nushi is a Japanese sea monster.

The story of the Yofune-nushi was recorded by Richard Gordon Smith in the book Ancient Tales and Folk-Lore of Japan, published in 1918. In the story, Yofune-nushi is a monster, living in the sea near the coast of one of the Oki Islands. It demanded a tribute of a virgin once a year. A brave girl, called Tokoyo, dives into the sea instead of the girl intended as an offering and defeats the unsuspecting monster. She also finds a cursed wooden idol that was causing a nobleman grief, thereby lifting the curse.