Merlin in the meantime was attentive to all that had passed, and then approached the king, and said to him, "For what reason am I and my mother introduced into your presence?"-- "My magicians," answered Vortigern, "advised me to seek out a man that had no father, with whose blood my building is to be sprinkled, in order to make it stand."-- "Order your magicians," said Merlin, "to come before me, and I will convict them of a lie." The king was surprised at his words, and presently ordered the magicians to come, and sit down before Merlin, who spoke to them after this manner: "Because you are ignorant what it is that hinders the foundation of the tower, you have recommended the shedding of my blood for cement to it, as if that would presently make it stand. But tell me now, what is there under the foundation? For something there is that will not suffer it to stand." The magicians at this began to be afraid, and made him no answer. Then said Merlin, who was also called Ambrose, "I entreat your majesty would command your workmen to dig into the ground, and you will find a pond which causes the foundations to sink." This accordingly was done, and then presently they found a pond deep under ground, which had made it give way. Merlin after this went again to the magicians, and said, "Tell me ye false sycophants, what is there under the pond." But they were silent. Then said he again to the king, "Command the pond to be drained, and at the bottom you will see two hollow stones, and in them two dragons asleep." The king made no scruple of believing him, since he had found true what he said of the pond, and therefore ordered it to be drained: which done, he found as Merlin had said; and now was possessed with the greatest admiration of him. Nor were the rest that were present less amazed at his wisdom, thinking it to be no less than divine inspiration.

Geoffrey of Monmouth's Historia Regum Britanniae