While Arthur’s men were listening to them putting their armor on and making ready, Merlin came out of his hiding place in the woods, for he was well aware of all their comings and goings; then he took a horn and blew it so loudly that the river and the whole woods resounded with it. After that he cast a strong and wondrous spell, for he made a great fire appear up in the air redder than a lightning bolt, and it was right above the tents of the enemies’ army. When King Arthur’s men saw the great fire burning in the sky and heard the horn blowing, they crossed themselves for the wonder they felt; then they let their horses run as fast as they could.

Vulgate Merlin