The Rhein Maidens (or Rhein Guardians) are the three immortals who devote their lives to guarding the invaluable metal, the Rheingold.
35 years later, a reformed Xena returned the ring to the Rhein Maidens. Surprised by the change in Xena, the youngest maiden asks what magic turned her into a force of good. Xena and Gabrielle exchange knowing looks as Xena explains, "It was no magic. . . ."
The Rhein Maidens are not mythological, but are instead inventions of Richard Wagner for his opera Der Ring das Nibelungen, which was combined with the Old English epic Beowulf to create the Norse storyline. In the story they were water nymphs.