Genesis 6:4
2 the sons of God saw that the daughters of humankind were beautiful. Thus they took wives for themselves from any they chose. 3 So the Lord said, “My Spirit will not remain in humankind indefinitely, since they are mortal. They will remain for 120 more years.” 4 The Nephilim were on the earth in those days (and also after this) when the sons of God would sleep with the daughters of humankind, who gave birth to their children. They were the mighty heroes of old, the famous men. 5 But the Lord saw that the wickedness of humankind had become great on the earth. Every inclination of the thoughts of their minds was only evil all the time. 6 The Lord regretted that he had made humankind on the earth, and he was highly offended.
Sirach 16:7
Ben Sira, Ecclesiasticus5 Many such things my eye has seen, and my ear has heard things more striking than these. 6 In an assembly of sinners a fire is kindled, and in a disobedient nation wrath blazes up. 7 He did not forgive the ancient giants who revolted in their might. 8 He did not spare the neighbors of Lot, whom he loathed on account of their arrogance. 9 He showed no pity on the doomed nation, on those dispossessed because of their sins;
Notes and References
"... More veiled allusions to punishment through the deluge may be reflected by the language of Sirach 16:7 (‘[God] did not forgive the giants of old’) and the Damascus Document (‘... and their sons whose height was as the height of cedars and whose bodies were as mountains [were caught] because they fell. All flesh which was on dry land decayed and became as if they never were.’). Other traditions allowed—similar to the Book of Watchers, Book of Giants, Jubilees - for the giants to persist beyond death, perhaps alongside the fallen angels, in a disembodied form of existence as spirits. This is presupposed in the sapiential songs document in 4Q510 and 4Q511, where the ‘spirits of the bastards’ are reckoned as powers with which the righteous still have to contend. In the later Christian Testament of Solomon (5:3; 17:1), the link between the post-diluvian demons and the giant offspring of the fallen angels is made explicit. This raises the possibility that would require more analysis than is possible here, namely, that mythological traditions about the giants may already have been adapted in passages of the gospels in which the desire of spirits to enter into human bodies is presupposed (so e.g. Mark 5:1–20; Matthew 12:43–45 / Luke 11:24–26) ..."
Stuckenbruck, Loren T. "The Origins of Evil in Jewish Apocalyptic Tradition: The Interpretation of Genesis 6:1-4 in the Second and Third Centuries B.C.E." in Auffarth, Christoph, and Loren T. Stuckenbruck (eds.) The Fall of the Angels (pp. 87-118) Brill, 2004