A story by La Motte Fouque,
published in German in 1811 and first translated ito English in 1818.

The water-sprite Undine, whose name the author took from Goethe, has been adopted and brought up by an old fisherman and his wife who have lost their own child. She meets the knight Huldbrand, who comes to the fisherman's cottage; the two fall in love and Undine marries him, only then revealing that she is not human. By the marriage she gains a soul. Huldbrand subsequently falls in love with Bertalda, who proves to be the fisherman's long-lost daughter. Undine return to her watery element, but when Huldbrand marries Bertalda the water-sprite reappears and kills the knight with a kiss.

The story, which has resemblences to The Little Mermaid by Andersen, is descended from Melusine, the French folk-tale of a water-sprite who who marries a knight on condition that he shall never see her on Saturdays, when she resumes her mermaid shape. Undine has been made ito a ballet and an opera. An unabridged English edition of the story published in 1909 has fine illustrations by Rackhan. George Macdonald thought Undine 'the most beautiful' of all fairy stories, and the references to it in such works as Charlotte Yonge's The Daisy Chain and Louisa Alcott's Little Women show that it was one of the best loved of all books for many 19th-century children.

Source: The Historical Mermaid