The story says here that when the Saxons and the giants were put to rout and driven from the fields, and the chase began, Merlin rode after a company of a good ten thousand men until he ran them down on a wide heath. Galahad, lord of the Land of the Grazing-Grounds, led that company, and as soon as they had come into the heath, Merlin cast a wondrous spell. He made a wide, roaring river well up before them in the way they had to go, and it fell so swiftly from the mountainsides that even the boldest among them took fright; and when they tried to turn back, they saw a fog coming in so thick and so fearsome that they did not know where to go, so they stayed there all night and did not go back or forward.

Vulgate Merlin