Sir Lancelot’s grandfather, King Lancelot, had a great and honorable love for his cousin’s beautiful, saintly wife. The cousin, a duke, misconstrued the relationship. As King Lancelot was on his way home through the Perilous Forest, he stopped to drink at a fountain. Here his cousin, the duke, ambushed him and cut off his head, which fell into the fountain. The fountain started to boil and scalded the duke’s hand when he tried to remove the head. The duke and his men buried the king near the fountain. As they were entering their castle, a stone fell from the roof and killed them.
King Lancelot’s widow erected a tomb. The tomb bled every day in several places, at the hour of the murder. Two lions fought fiercely over a stag near the tomb. They wounded each other grievously, but the drops of blood from the tomb healed them. Henceforth, they guarded the tomb, taking turns to hunt their food. A hermitage, also, was either near this site already or was built nearby in the years between King Lancelot and his more famous grandson.
When Sir Lancelot came to the well, having been directed by his grandfather in a dream, he killed the lions, retrieved his grandfather’s head, opened the tomb, and, with the help of the hermit, reburied the body, with the head, at the front of the altar, where King Lancelot’s wife was buried. Because Sir Lancelot was not pure, however, the water continued to boil. Only when Galahad arrived did the water cease boiling; the fountain was thereafter called Galahad’s Fountain, and it boiled constantly.
Malory omits the above history, but describes Galahad’s coming to the place. Galahad departed from the “Abbey of King Mordrains”
and so came into a perilous forest where he found the well the which boileth with great waves. ... And as soon as Galahad set his hand thereto it ceased, so that it brent no more, and the heat departed. For that it brent it was a sign of lechery, the which was that time much used. But that heat might not abide his pure virginity. And this was taken in the country for a miracle. And so ever after was it called Galahad's Well.
Because Malory’s work seems largely a summarization, that Malory recounts Galahad’s visit to the boiling well immediately after his visit to Mordrain’s abbey does not necessarily mean they are in the same region. The Vulgate tells us that Lancelot lost his way while returning from this forest to Le Tertre Deuee and saw the white stag conducted by four lions. This appears to place the “Forest of the Boiling Well,” the castle of Le Tertre Deuee, the abbey of La Petite Aumosne, and possibly King Vagor’s Isle Estrange in the general vicinity of Carteloise Forest in southern Scotland.
See also
Chapel of the Boiling Well | The Legend of King Arthur
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470