Swan maiden
The "swan maiden" (German: Schwanjungfrau) is a tale classified as ATU 400, "The Swan Maiden" or "The Man on a Quest for His Lost Wife", in which a man makes a pact with, or marries, a supernatural female being who later departs.
Source: Wikipedia, Swan maiden. Text licensed CC BY-SA 4.0. Not endorsed by or affiliated with Wikipedia or the Wikimedia Foundation.