Gwragedd annwm (sing: Gwraig) are Welsh lake maidens.
Behavior
Gwragedd annwm are said to be lovely and gentle, and rarely dangerous like so many other water fairies. Occasionally mortals would entice them into marriage, but as usual there was always a taboo associated the marriage.
Tale
The Gwraig and the Three Blows
A young farmer fell in love with a water maiden who used to row about the lake in a golden boat, singing. He saw her close at hand by watching the New Year in at the edge of the lake, and courted her by floating gifts of bread and cheese upon the water. He did this from Midsummer Eve till the New Year, when she again appeared to him, and consented to be his wife, but warned him that if he struck her three times she must leave him. They lived happily for a long time, and she was a loving and dutiful wife, though she had strange ways.
Once they went to a child's christening, and she would do nothing but weep. Her husband tapped her reprovingly, and she said to him: "How can I do else but weep, when the poor child is born into a world of sorrow? And that is the first blow."
A little later the baby died, and they went to its funeral. Here, where everyone else was sad, she danced and sang. Her husband tapped her again. "The Baby is gone from temptation to be happy for ever," she said. "How can I help rejoicing? Be careful, that is the second blow."
Some time later they went to a wedding, where a beautiful young girl was wedded to an old miser, and in the middle of the feast the gwraig burst into tears, and cried out: "Beauty is wedded to age, not for love but for greed. It is the devil's compact!" Angry and scandalized, her husband struck her, and said "Be quiet." She looked with sorrowful love at him, and said: "It is the third blow, I must leave you." She vanished as she spoke, and the rich dowry of fairy cattle she had brought went with her.
Source
Sikes, British Goblins, pp. 40.