The Tylwyth Teg (Welsh: "the Fair Folk") are elves similar to Bendith y Mamau in the folklore of Wales.
In many accounts, their king is said to be Gwyn ap Nudd. Types include 'Jili Ffrwtan,' 'Coblynau,' and 'Ellyllon' amongst many others.
Until the early 19th century it was commonly believed that the Tylwyth Teg, described as ethereal, beautiful and fair-haired, dwelt in a number of places in Wales as genii loci similar to Greek nymphs, Norse norns or Irish sidhe. Such places included the lake Llyn y Fan Fach. Tylwyth Teg had Fairy paths upon which it was dangerous for a mortal to walk.
The Tylwyth Teghey are usually portrayed as benevolent but capable of mischief, neither entirely good nor completely evil, unlike the Scottish division into Seelie and Unseelie. In their benevolent capacity they might, for example, reward with gifts of silver a woman who kept a tidy house. Yet the Tylwyth Teg might also leave a changeling child in place of a human baby. They are said to covet beautiful mortal children. They fear iron, so unbaptized children could supposedly be protected from them by placing a poker over their cradle.
In ancient times a door in a rock near this lake was found open upon a certain day every year. I think it was May-day. Those who had the curiosity and resolution to enter were conducted by a secret passage, which terminated in a small island in the centre of the lake. Here the visitors were surprised with the prospect of a most enchanting garden stored with the choicest fruits and flowers, and inhabited by the Tylwyth Teg, or Fair Family, a kind of Fairies, whose beauty could be equalled only by the courtesy and affability which they exhibited to those who pleased them.
They gathered fruit and flowers for each of their guests, entertained them with the most exquisite music, disclosed to them many secrets of futurity, and invited them to stay as long as they should find their situation agreeable. But the island was secret, and nothing of its produce must be carried away. The whole of this scene was invisible to those who stood without the margin of the lake. Only an indistinct mass was seen in the middle; and it was observed that no bird would fly over the water, and that a soft strain of music at times breathed with rapturous sweetness in the breeze of the morning.
It happened upon one of these annual visits that a sacrilegious wretch, when he was about to leave the garden, put a flower, with which he had been presented, in his pocket; but the theft boded him no good. As soon as he had touched unhallowed ground the flower vanished and he lost his senses. Of this injury the Fair Family took no notice at the time. They dismissed their guests with their accustomed courtesy, and the door was closed as usual.
But their resentment ran high. For though, as the tale goes, the Tylwyth Teg and their garden undoubtedly occupy the spot to this day, though the birds still keep at a respectful distance from the lake, and some broken strains of music are still heard at times, yet the door which led to the island has never re-opened, and from the date of this sacrilegious act the Cymry have been unfortunate.
Some time after this, an adventurous person attempted to draw off the water, in order to discover its contents, when a terrific form arose from the midst of the lake, commanding him to desist, or otherwise he would drown the country.