Geoffrey of Monmouth describes how the young Merlin nearly became a blood sacrifice to propitiate the building of Vortigern's castle, whose foundations collapsed every night. Merlin challenged Vortigern's wizards to divine the collapse, predicting that beneath the fortress was a large pool with two hollow stones, each containing a sleeping dragon. When the pool was drained the dragons, one white and one red, were found and they fought with first the white one having the advantage before the red recovered and drove it back.

Merlin explained that the red dragon represented the Britons and the white the Saxons and prophesied that Vortigern, the usurper, would be slain by Britain's rightful princes, Aurelius Ambrosius and Uther. When this later happened he became known as the bard of Vortigern.