Hans Christian Andersen's tale of "The Marsh King's Daughter" (1858) follows Helga, the daughter of a monster and a kidnapped princess. During the day she is beautiful like her mother but violent and cruel; during the night, she is hideous like her father but sad and gentle. There are heavy Christian themes, with Helga meeting a noble missionary priest and breaking free of her curse through the power of God. At the end, Helga and her mother return to Egypt. Helga is about to be married to a prince, but seems distracted from her impending wedding. She has spent a lot of time meditating on Christianity and the now-dead priest who saved her. She prays for a glimpse of Heaven and is allowed to see its glory for three minutes. When she returns, however, she learns that "many hundred years" have passed since she vanished on her wedding day. Upon hearing this, her body crumbles to dust, freeing her to return to Heaven.
This story always pulled me in at the beginning with its concept and descriptions, but the ending was just depressing. Yes, Helga’s greatest desire is to go to heaven, but I still found the ending dissatisfying and discomfiting. There's just something freaky about your heroine going all Infinity War at the end. And I say this as someone who grew up loving stories of martyrs and saints.
Andersen, as usual, pulled in a lot of fairy tale concepts. The beginning is very familiar, with a group of swan maidens taking off their feathery cloaks to bathe, and a man who captures and forcibly marries one of them. However, this trope usually features a human man winning a supernatural bride. In this case, the swan maidens are human princesses and the man is a literal swamp monster.
And the ending of Helga's story is a popular medieval legend. In fact, that legend is a fairy motif repackaged by Christian storytellers. This motif has been incredibly widespread from ancient times up to modern literary tales like Rip Van Winkle.
Urashima Tarō, a Japanese tale dating back to the 8th century, centers around a fisherman named Urashima who catches a turtle which turns out to be a princess of the sea. She takes him away to her blissful underwater kingdom, where he has eternal life and everything he could ever want. What he wants, though, is to visit his old home on land. He arrives only to find that centuries have passed and everything he knew is gone. The ending varies, but generally all of his years come upon him at once and he is left an old man, his immortality gone. And in The Voyage of Bran from Ireland, also from around the 8th century, much the same thing happens. After seeing a beautiful silver branch, Bran sets out for the Land of Women, the utopian island where the branch grew. He and his men live there for what seems like a year, feasting and totally happy, but one of them feels homesick. The band returns to Ireland briefly, and learns that centuries have passed and they are remembered only as legends. The homesick man steps onto dry land and turns to dust, and his companions decide they'd better book it back to the Land of Women.
King Herla, an English character from the 12th century, had the same experience after dealing with a dwarf king. And in a 12th- or 13th-century lai, the knight Guingamor (just like Urashima and Bran) immediately regrets leaving his supernatural sweetheart.
The moral in Urashima and Bran's tales is to not break taboos. In both cases, a man ignores the commands of his lover (who is basically a goddess) and dooms himself to a terrible punishment. King Herla's post-Christian story has the moral that the supernatural creatures of older religions are treacherous and evil. Herla is punished for having anything to do with the fae.
In medieval times, the story got repurposed. The land of joy and immortality was replaced by a Christian Paradise. Often, the hero of this story was a monk or bishop. The story was used to illustrate the idea that Heaven is so wonderful that a thousand years there are like three minutes, and earthly life is nothing compared to it. The main character would return long enough for people to confirm his identity and be amazed by the miracle, before he disintegrates and joyfully returns to Heaven for good. The main idea of the story is that eternity will not be boring, an issue which has apparently nagged at people for a long time.
Versions appeared in English, Spanish, Slovenian, you name it. A fourteenth-century Italian legend featured four monks who, like Bran, went off seeking Paradise after finding a wondrous tree branch from that location (MacCulloch, Medieval Faith and Fable, p. 199). The most widespread version, where a bishop is entranced by the song of an angelic bird, appeared in a homily by the 12th-century French bishop Jacques de Vitry.
At the same time, interestingly, the story has survived with fairy roots intact. For instance, a Welsh story of a farmboy who sits under a tree listening to a bird’s entrancing music parallels the story of the bishop. Despite the similarities, it's clear that the bird in the Welsh fairytale is from a very different otherworld than Heaven. (Howells, Cambrian Superstitions)
Hans Christian Andersen may have been particularly inspired by something close to home: the Danish tale of "The Aged Bride." Published in Benjamin Thorpe's Northern Mythology (1851), it follows a bride who steps out of a dance at her wedding and notices elves celebrating in a nearby field. When she approaches, they offer her wine and invite her to join in their dance. Completing the dance, she remembers her husband and hurries home. There, however, she finds herself in a situation identical to Helga's, Urashima's, and Rip Van Winkle's. The wedding party has vanished and the town looks completely different. No one recognizes her except as an old story from a hundred years ago. Upon hearing this, she falls down dead. Compared to this story illustrating the dangers of the fairy world, "The Marsh King's Daughter" is positively cheery.
Researching folktales and fairies, with a focus on common tale types.