On the origins of the race of mermaids and mermen Icelandic folklore has is this story:
Once upon a time there was a king and his queen who had one daughter, an only child. When she was born the witches and sorceresses of the land gave her many good blessings, including beauty, riches and the promise of a good husband. But one of the witches was annoyed and jealous because of all the gifts heaped upon the child and cursed her, saying that she would turn into a sea-monster every ninth night after her marriage and she should live like this for three years so that none but her husband could find out and if he did find out, her freedom from the curse would depend upon him not revealing it to anyone, not being angered by it or stop loving her. But if one or more of these stipulations was not fulfilled, she must forever live in the sea and never be freed. But as some consolation she would have a such beautiful singing voice that all creatures who heard it would be soothed into sleep by it.
The princess grew up in her father’s palace, beautiful and happy and beloved by all, and when she was grown she was given in marriage to a fine prince, and then the curse struck. For two years she was able to sneak undetected from their bed every ninth night, but during the third year the prince noticed her frequent disappearances and started keeping a closer eye on her.
One night he followed her down to the sea and into a cave. When he came to the cave he saw her swimming in the water down below, with the tail of a fish from the legs down but completely human above. He then returned to the palace and went back to bed, but when she returned and tried to get into bed with him, he angrily called her a “disgusting worm” and ordered her to leave and never come back. So she returned to the sea, crying and sad and never stepped back on land. But she was pregnant when she left and gave birth to her babies in the sea, and from them all the merpeople are descended.
She is sometimes mentioned in old stories and is said to like putting men to sleep with her singing, and that she can sing any song except the Te Deum. Once she is said to have swum behind a ship, singing so beautifully that all the crew fell asleep, except one man who was an exceptional singer himself and sang all the songs with her. Finally he sang the opening of the Te Deum, but she could not sing it so she dived down and swam away.