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The Undines are elementals of water. They look like women though they sometimes take the form of a fish or a snake. They have cold, soft skin and their beautiful voices can be heard over a rushing waterfall. They live in forest pools, lakes, and streams.
Undines can take on a completely human form to marry a human man. In some stories, they are motivated to do this to gain a human soul. They are notoriously vengeful against a human lover who betrays them. For instance, a German myth said that an "Ondine" fell in love with a knight who swore to be faithful to her with every waking breath. When she found him asleep in the arms of another woman, she cursed him so that if he ever fell asleep again, his breath would be taken from him and he would die.