Genesis 6:1, The Sons of God Mated With the Daughters of Man.

        Just what this meant to the priest of the 6^th century B.C. this tale is unsure, for it was part of a larger unpreserved account, unless the Hebrew Pseudepigrapha and Gnostics on the Creation are to be considered its progeny.  Let us begin with Genesis 6:1:

When men began to multiply on earth and daughters were born to them, the sons of heaven saw how beautiful the daughters of man were, and so they took for their wives as many of them as they chose.  Then the Lord said:  My spirit shall not remain in man forever, since he is but flesh.  His days shall comprise one hundred and twenty years. 

At that time the Nephilim appeared on earth (as well as later), after the sons of heaven had intercourse with the daughters of man, who bore them sons.  They were the heroes of old, the men of renown. 

         Later men of faith provided details consistent with other parts of the Old Testament.  In fact there was a large collection of these mythic tales from Medieval times that have come down to us, and a number from the Roman and Greek times.  What follows are two Jewish accounts, their earliest date unknown.[1] 

        The story at the beginning of Genesis 6 is now regarded as an unassimilated fragment from an alien myth,[2] but its influence on Jewish and Christian thought has been very great. In this the sons of God were enamoured of daughters of men, who ore to them men of renown.  In later Jewish exegesis [my emphasis] these sons of God descend from Seth, Adam's third and favourite son, and the daughters of men from the evil line of Cain, or sometimes they are just good young men and girls of easy morals. But there can be no doubt that this is originally a story about angels.  In one version they came down as a body upon Mount Hermon, where two hundred of them under twenty captains swore to maintain their common purpose of rebellion against God. They then seduced the girls and taught them charms and enchantments, especially the virtues of roots and of switches from particular trees that have always played an important part in witchcraft. Their enchanted ladies bore them children, who soon became enormous and grew up to be supermen whose appetites consumed all that their human cousins had gathered, and then they turned to cannibalism.

In another version of the story the angels came to earth by divine permission before the flood, [the next story in Genesis is of the flood] when evils were increasing and men were going irretrievably to the bad. They offered to do something to save the situation, but God warned them that if they went to live among men their own inclinations would turn to evil and overcome them. They promised to do every­thing to sanctify Gods holy name, and were allowed to go. They made friends with the women, including one good and intelligent girl who wormed the ineffable name of God out of one of them, and so got away to heaven, where she became the constellation of the Pleiades. But others of the women were in competition for the supplies of rouge and other ornaments provided by Azazel,[3] the most ingenious of the angels, and themselves took the initiative in drawing him and others into more dangerous forms of physical contact. Finally they produced babies, some of whom grew into giants, as in the other tale. The giants included the Emin, the Gibborim, the Anakim, and the Nephilim; at least one, Ashmedai or Asmo­deus, was a real devil, who strangled new born children if he was allowed to get at them. His mother was Naamah, the sister of Tubal-cain, who was the first to make instruments of bronze and iron. In this way at least the seed of the fallen angels is linked with the line of Cain, and with the growth of human ingenuity in the direction of power to murder.

In the Dead Sea Scroll are the fragments of The Book of Giants.  Though possible 5 centuries later than the Genesis text, it is invaluable.