Undine (alchemy) - Sleep of Ondine

In a German tale known as Sleep of Ondine, Ondine is a water nymph. She was very beautiful and, like all nymphs, immortal. However, should she fall in love with a mortal man and bear his child, she would lose her immortality.

Ondine eventually falls in love with a handsome knight, Sir Lawrence, and they are married. When they exchange vows, Lawrence vows to forever love and be faithful to her. A year after their marriage, Ondine gives birth to his child. From that moment on she begins to age. As Ondine’s physical attractiveness diminishes, Lawrence loses interest in his wife.

One afternoon, Ondine is walking near the stables when she hears the familiar snoring of her husband. When she enters the stable, she sees Lawrence lying in the arms of another woman. Ondine points her finger at him, which he feels as if kicked, waking him up with surprise. Ondine curses him, stating, "You swore faithfulness to me with every waking breath, and I accepted your oath. So be it. As long as you are awake, you shall have your breath, but should you ever fall asleep, then that breath will be taken from you and you will die!"