Undines, sometimes spelled Ondine, are a water nymph who becomes human when she falls in love with a mortal man. If he is unfaithful, her death is inevitable.
Derived from the Latin word unda, which means wave or water.
Undines were first discussed by Paracelsus (circa 1493 – 1541). They are believed to be connected to Greek mythology figures known as Nereids, who were portrayed as young women who lived in any body of water and were kind to humans.
A story of an Undine tells of how, in a fishing village, a human couple had lost their own child but shortly after found a baby left at their door. They took her in as their own, and she grew to a most beautiful young woman with pearly skin and green eyes, both loving and fickle in her nature.
Hildebrand saw and fell in love with her and took her as his wife. But he betrayed her with another named Bertalda. With this breaking of the vow, Undine was reclaimed by her Merfolk and vanished back to the sea.
However, on the eve of his wedding to Bertalda, Hildebrand went to the well in the courtyard and there he saw Undine. She embraced him and took his soul with her to the waters, leaving his body by the well.Carol Rose – Spirits, Fairies, Gnomes, and Goblins