In the year 1180 CE a young farmer met a beautiful woman from the land of the fairy. They fell in love and married. Their marriage was with one condition; He must never strike her three times without cause Tri ergyd di-achos He readily agreed to this condition for he felt that he could never strike this woman. However, as so often happens when such a condition is made, he does strike those three blows. The woman, whose name is given as Nelferch, returns to her world and the husband never sees her again.
What makes this tale particularly interesting and unique among such tales is what happens after the tale ends. The couple had three sons and though she never again appeared to the husband, she did for her children. She took them around the Black mountains of Wales, showing them herbs and explaining the healing properties of those herbs. She taught them the arts of medicine until the three sons became famous for their healing skills. Their descendants continued in this tradition until Dr. Rhys Williams M.D. died in 1842. It is said that he was the last of the descendants of the lady of the lake.
Fortunately, the children of Nelferch wrote down the knowledge they had gained and a copy of it can be seen at Jesus College, Oxford. A translation of "The Medical Book of the Physicians of Myddfai" can be accessed online here; http://www.maryjones.us/ctexts/meddigion_myddfai.html