One day they happened to come into a large, beautiful wooded area and saw in the middle of the road a rock as high as you could see with your eyes. They went in that direction to see the rock, for it seemed to them a marvel. When they were near, they saw neither step nor path by which one might go up, and the rock was so steep and smooth that it would be a wonder if a squirrel could climb it.

When they had gone around it on all sides, they said to each other, “God, why was such a thing made, when there’s neither house nor shack where men could shelter?”

As they were talking, they looked up to the height and saw as many as twelve young ladies, the loveliest and best dressed they had seen in a long time.

[...]

Then Morholt began to think, and Sir Gawain asked him what he was thinking.

“I’m thinking,” he said, “about these maidens, and I know who they are and why they are there. There are twelve of them, full sisters, and the eldest knows a wonderful amount about spells. Because of her great knowledge, she set herself against Merlin and thought to kill him by her spells, because he often thwarted her in things she wanted to do. Then Merlin, who knew more than she did, thought he would avenge himself on her for desiring his death; I don’t know what he did to her by spells, but at the end he put her and her sisters there, on top of this rock, and he got her there by the devil’s arts. Be sure they would have died in a short time, but that couldn’t be, for she who is there knows so much of magic that if there were but one loaf of bread in the world, and it were one hundred days’ journey distant, she would make it come to her in one day.”

Post-Vulgate Merlin