Some time after, wishing to construct an impregnable fortress on Salisbury Plain, Vortigern sent for a host of masons, who were dismayed to see the work they had done during the day destroyed every night.
On consulting an astrologer, Vortigern was directed to anoint the stones with the blood of a boy of five who had no human father.

The only child corresponding to this description was Merlin, who saved himself from untimely death by telling the king that, if he dug down and drained the lake he would find, he would discover broad stones beneath which slept two dragons by day, although they fought so fiercely at night that they caused the tremendous earthquakes which shattered his walls.

These directions were followed, the dragons were roused, and fought until the red one was slain and the two-headed white one disappeared.

Asked to explain the meaning of these two dragons, Merlin--the uncanny child--declared the white dragon with two heads represented the two younger sons of King Constance, who were destined to drive Vortigern away.

Having said this, Merlin disappeared, thus escaping the wrath of Vortigern, who wished to slay him.
Soon after, the young princes surprised and burned Vortigern in his palace, and thus recovered possession of their father's throne.