Undine

undine, also spelled Ondine,  mythological figure of European tradition, a water nymph who becomes human when she falls in love with a man but is doomed to die if he is unfaithful to her. Derived from the Greek figures known as Nereids, attendants of the sea god Poseidon, Ondine was first mentioned in the writings of the Swiss author Paracelsus, who put forth his theory that there are spirits called “undines” who inhabit the element of water.