King Arthur and his Knights by Thomas Bulfinch

Merlin was the son of no mortal father, but of an Incubus, one of a class of beings not absolutely wicked, but far from good, who inhabit the regions of the air. Merlin's mother was a virtuous young woman, who, on the birth of her son, entrusted him to a priest, who hurried him to the baptismal fount, and so saved him from sharing the lot of his father, though he retained many marks of his unearthly origin.

At this time Vortigern reigned in Britain. He was a usurper, who had caused the death of his sovereign, Moines, and driven the two brothers of the late king, whose names were Uther and Pendragon, into banishment. Vortigern, who lived in constant fear of the return of the rightful heirs of the kingdom, began to erect a strong tower for defence. The edifice, when brought by the workmen to a certain height, three times fell to the ground, without any apparent cause. The king consulted his astrologers on this wonderful event, and learned from them that it would be necessary to bathe the cornerstone of the foundation with the blood of a child born without a mortal father.