The origin of a mermaid.
This story was inspired by a prompt on Tumblr.
In the past, sailors would throw women overboard, since women were signs of bad luck. But they didn't know that women thrown overboard, with their legs bound to prevent them from swimming, are saved by the ocean. Slowly, their legs fused together to form a beautiful tail, and the water changed them to become fairer and a goddess of the ocean. Then, they sought the ship that had betrayed them and sing the sailors to their deaths.