In reward for his defection at this critical moment, Vortigern was offered the crown, which he accepted, and which he hoped to retain, although Constans's two other sons, who, according to another version of the story, were called Uther and Pendragon, were still in existence.
To defend himself against any army which might try to deprive him of the throne, Vortigern resolved to build a great fortress on the Salisbury plains.

But, although the masons worked diligently by day, and built walls wide and thick, they always found them overturned in the morning.

The astrologers, when consulted in reference to this strange occurrence, declared that the walls would not stand until the ground had been watered with the blood of a child who could claim no human father.
Five years previous to this prediction, the demons, seeing that so many souls escaped them owing to the redemption procured by a child of divine origin, thought that they could regain lost ground by engendering a demon child upon a human virgin.

A beautiful, pious maiden was chosen for this purpose; and as she daily went to confess her every deed and thought to a holy man, Blaise, he soon discovered the plot of the demons, and resolved to frustrate it.
[Sidenote: Birth of the mythical Merlin.] By his advice the girl, instead of being immediately put to death, as the law required, was locked up in a tower, where she gave birth to her son.

Blaise, the priest, more watchful than the demons, no sooner heard of the child's birth than he hastened to baptize him, giving him the name of Merlin.