In the middle of the episode "Last Resort" Serena's mother is tells her the Legend of the Guardians of the Lake. She tells her that a young man fell in love with a water sprite, but a woman from his village also loved him, and was so jealous of their love, she turned into a monster. Then the man and water sprite concealed her into the lake. Here's the slip up, Serena's mother ends the tale with, "And the jealous sprite was doomed to be miserable forever..." The monster was a woman, not the sprite.