Undines are water spirits, first named in the alchemical writings of Paracelsus. In the tale by Friedrich de la Moote Fouqué, such a spirit marries a night, Huldebrand, in order to gain a soul. It was a popular book in the nineteenth century and illustrated by Rackham in 1909. The underlying French folk tale is called Melusine, wherein a water sprite marries a knight on the promise that he never see her on Saturdays, when she retakes her mermaid form.
He held up the gold piece, crying at each leap "False Gold! False Gold!"