1 Enoch 15:4
2 And go, say to the Watchers of heaven, who have sent you to intercede for them: "You should be interceding for men, not men for you:" 3 Why have you left the high, holy, and eternal heaven, and lain with women, and defiled yourselves with the daughters of men, taken yourselves wives, and done like the children of earth, and fathered giants as your sons? 4 And though you were holy, spiritual, living the eternal life, you have defiled yourselves with the blood of women, and fathered children with the blood of flesh, and, like the children of men, have lusted after flesh and blood as those also do who die and perish. 5 Therefore I have given them wives also that they might impregnate them, and beget children by them, that thus nothing might be lacking to them on earth. 6 But you were formerly spiritual, living the eternal life, and immortal for all generations of the world.
Jubilees 7:21
21 For it was on account of these three things that the flood was on the earth, since it was due to fornication that the Watchers had illicit intercourse — apart from the mandate of their authority — with women. When they married of them whomever they chose, they committed the first acts of uncleanness. 22 They fathered as their sons the Nephilim. They were all dissimilar from one another and would devour one another: the giant killed the Naphil; the Naphil killed the Elyo; the Elyo mankind; and people their fellows. 23 When everyone sold himself to commit injustice and to shed innocent blood, the earth was filled with injustice.
Notes and References
"... The ‘Sons of God’ of Genesis 6:1-4 are described in the Pseudepigraphical Jewish literature as being - ‘evil angels’, ‘demons’ (Jubilees 10:2), ‘Watchers’ (1 Enoch 10-16; Jubilees 7:21; 10:5; and Testament of Naphtali 3:5), ‘stars’, ‘spirits’ (pneumata, see 1 Enoch 15:4-10, 16:1 and Jubilees 10:3, 7) and ‘spirits of the angels’ (1 Enoch 19:1). Their illegitimate offspring, the Nephilim, are also described as ‘(evil) spirits’ (1 Enoch 15:8-12; 16:1 and Jubilees 10:3-5) or ‘demons’ (Jubilees 7:22; 10:1f) ..."
Campbell, D.N. & van Rensburg, F.J. A History of the Interpretation of 1 Peter 3:18-22 (pp. 73-96) Acta Patristica et Byzantina 19(1), 2008