Undine

Undines /ˈʌnˌdiːn, ˌʌnˈdiːn/ (or ondines) are a category of elemental beings associated with water, first named in the alchemical writings of Paracelsus.

Undines are almost invariably depicted as being female (though some are male), and are usually found in forest pools and waterfalls. The group contains many species, including nereides, limnads, naiades and mermaids. Although resembling humans in form, they lack a human soul, so to achieve immortality they must acquire one by marrying a human. Such a union is not without risk for the man, because if he is unfaithful, then he is fated to die.

Characteristics
Undines are almost invariably depicted as being female, which is consistent with the ancient idea that water is a female element, though some are male. They are usually found in forest pools and waterfalls, and their beautiful singing voices are sometimes heard over the sound of water. The group contains many species, including nereides, limnads, naiades, mermaids and potamides.

What undines lack, compared to humans, is a soul. Marriage with a human shortens their lives on Earth, but earns them an immortal human soul.

The offspring of a union between an undine and a man are human with a soul, but also with some kind of aquatic characteristic, called a watermark. The man may have a cleft in his throat, for instance, which must be periodically submerged in water to prevent it from becoming painful.

Etymology
Undine is a term that appears in the alchemical writings of Paracelsus, a Renaissance alchemist and physician. It is derived from the Latin word unda, meaning "wave", and first appears in Paracelsus' book Liber de Nymphis, sylphis, gnomes et salamandris et de caeteris spiritibus, published posthumously in 1658. Ondine is an alternative spelling, and has become a female given name.

Ondine's Curse
Congenital central hypoventilation syndrome, a rare medical condition in which sufferers lack autonomic control of their breathing and are hence at risk of suffocation while sleeping, is also known as Ondine's curse. Ondine, the eponymous heroine of Giradoux's play, tells her future husband Hans, whom she has just met, that "I shall be the shoes of your feet ... I shall be the breath of your lungs". Ondine makes a pact with her uncle, the King of the Ondines, that if Hans ever deceives her he will die. After their honeymoon Hans is reunited with his first love, the Princess Bertha, and Ondine leaves him, only to be captured by a fisherman six months later. On meeting Ondine again on the day of his wedding to Bertha, Hans tells her that "all the things my body once did by itself, it does now only by special order ... A single moment of inattention and I forget to breathe". Hans and Ondine kiss, and he dies.