The Carmarthenshire village of Myddfai could perhaps be the birthplace of modern medicine. According to legend, a dynasty of herbalists known as the Physicians of Myddfai lived and worked here in the 11th and 12th centuries. Some believed they had magical powers.

The 14th century Red Book of Hergest, one of the most ancient manuscript volumes in existence, contains a tale set which begins beside Llyn-y-Fan Fach, the lake below the peak of Black Mountain in the west of the Brecon Beacons National Park. The same tale is recorded in Mabinogion, a collection of Welsh folktales that date back to medieval times.

According to the story, a Carmarthenshire farmer once saw a beautiful woman sitting on a rock in Lyn-y-Fan Fach. After three attempts to woo her, she agreed to marry him if he promised to treat her well. If he were to strike her three times without cause, she said, she would return to the lake.

The farmer made his promise and took her down to the village of Myddfai to live with him as his wife.

The time came for the christening of their first child. The farmer was overjoyed, but the lady cried because, through her magical instincts, she knew the baby would be harmed by the sun. Misunderstanding, the farmer tapped her lightly to bring her round.

Later, she cried at a wedding because she knew the bridegroom would soon die. Again, the farmer tapped her lightly to bring her to her senses.

Finally, she laughed at the bridegroom's funeral because she knew his suffering was over and she was happy for him. And the farmer tapped her again.

Immediately, the lady sped back to the lake. The farmer, powerless to stop her, was left heartbroken, destined to bring up their three sons alone.

As the sons grew up, it became clear that they had inherited their mother's magical knowledge and powers. They could have used these to become great warriors, but chose instead to become the first in a long line of great healers.

Using natural products gathered from the surrounding area, the Physicians of Myddfai created cures and remedies for headaches, sunburn, swellings, coughs and sneezes. Some of these ancient remedies are recorded in the Red Book of Hergest. The book itself currently belongs to Jesus College, Oxford, and is kept in the Bodleian Library.

Some believe that the myth of Llyn-y-Fan Fach gave rise to another famous tale – the Arthurian legend of the Lady of the Lake and Excalibur.