Mari-Morgan

Mari-Morgens are believed to live near coasts, at cave entrances and at the mouths of rivers. They are well-versed in evil spells, and drag young men underwater never to be seen again.
They are said to carry kidnapped sailors to underwater palaces of mother-of-pearl and crystal, and marry them. The Mari-Morgens, eternally young, are also blamed for heavy flooding that destroys crops or villages.
The princess Dahut, who betrayed the city of Ys and caused it to flood, was transformed into a Mari-Morgen as punishment.