Mythological Ondines are water nymphs, nereides, limnads, naiades and mermaids – all beings associated with water. Although resembling humans in form they lack a human soul, but she can acquire one by marrying a human. Such a union is not without risk for the man, because if he is unfaithful he is fated to die.
Those under the Curse must always remember to breathe in and breathe out. Forgetting to do this would result in death.
It became popular by a romance Undine by Friedrich de la Motte Fouqué, published in 1811,which was based on a passage in Paracelsus’ Liber de Nymphis in which he relates how an ondine can acquire an immortal soul by marrying a human.
Arthur Rackham, illustration from the book by Friedrich de la Motte Fouqué “Undine” from 1909