Genesis 6:4
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.
1 Enoch 7:2
1 And all the others them took wives for themselves, each choosing one for himself, and they began to unite with them and defiled themselves with them. They taught them sorcery, spells, and the art of extracting medicinal substances from plants. 2 The women became pregnant and gave birth to enormous giants, whose height was three thousand ells. 3 These giants consumed everything humans produced. And when humans could no longer support them, 4 the giants turned against them and devoured mankind.
Baruch 3:26
24 O Israel, how great is the house of God, how vast the territory that he possesses! 25 It is great and has no bounds; it is high and immeasurable. 26 The giants were born there, who were famous of old, great in stature, expert in war. 27 God did not choose them, or give them the way to knowledge; 28 so they perished because they had no wisdom, they perished through their folly.
Notes and References
"... 1 Enoch 7:3-5 ... In later literature, the giants' wickedness and God's punishment of it become proverbial, more so than the example of their fathers, the watchers. In Jubilees 7:22-23 the present passage is paraphrased with an emphasis on their violence. Sirach 16:7 speaks of their rebellion. They are one of a series of examples of God's not pardoning or showing pity to sinners. Similarly, 3 Maccabees 2:4 mentions them (as in Sirach 16:7, followed immediately by Sodom) in a catalog of sinners who were punished. Another such catalog of sinners appears in the Damascus Document 2:17-3:12. The giants and their fathers are said to have fallen, perhaps a play on the name nephilim. Wisdom of Solomon 14:6 mentions in passing that they were arrogant. In 1 Baruch 3:26-28 the giants are the example par excelence, and the only example, of folly .. the passage appears to depend both on Genesis 6:4 and the Enochic version with its reference to their destruction ..."
Nickelsburg, George W. E. A Commentary on the Book of 1 Enoch Chapters 1-36, 81-108 (p. 186) Fortress Press, 2001