Here are a few key segments from the Book of Enoch which delve into the topic of the Nephilim:
Enoch 6:6 - iii, 3 Excellent lengthy account of the angelic fallen Watchers which for brevity purposes you'll have to research for yourselves.
1 Enoch 6:1 1 And when the sons of men had multiplied, in those days, beautiful, and comely daughters were born to them. 2 And the watchers, the sons of heaven, saw them and desired them. And they said to one another, “Come, let us choose for ourselves wives; from the daughters of men, and let us beget for ourselves children.” 3 And Shemihazah, their chief, said to them, “I fear that you will not want to do this deed, and I alone shall be guilty of a great sin.” 4 And they all answered him and said, “Let us all swear an oath, and let us all bind one another with a curse, that none of us turn back from this counsel until we fulfill it and do this deed.”
1 Enoch 6:6 6 And they were, all of them [watchers], two hundred, who descended in the days of Jared onto the peak of Mount Hermon. And they called the mountain “Hermon” because they swore and bound one another with a curse on it.
1 Enoch 7:1 1 These and all the others with them took for themselves wives from among them such as they chose. And they began to go in to them, and to defile themselves through them, and to teach them sorcery and charms, and to reveal to them the cutting of roots and plants. 2 And they conceived from them and bore to them great giants. … And they were growing in accordance with their greatness. 3 They were devouring the labor of all the sons of men, and men were not able to supply them. 4 And the giants began to kill men and to devour them. 5 And they began to sin against the birds and beasts and creeping things and the fish, and to devour one another’s flesh. And they drank the blood. 6 Then the earth brought accusation against the lawless ones.
1 Enoch 8:1 1 “Asael taught men to make swords of iron and weapons and shields and breastplates and every instrument of war. He showed them metals of the earth and how they should work gold to fashion it suitably, and concerning silver, to fashion it for bracelets and ornaments for women. And he showed them concerning antimony and eye paint and all manner off precious stones and dyes. And the sons of men made them for themselves and for their daughters, and they transgressed and led astray the holy ones. 2 And there was much godlessness upon the earth, and they made their ways desolate. 3 Shemihazah taught spells and the cutting of roots. Hermani taught sorcery for the loosing of spells and magic and skill Baraqel taught the signs of the lightning flashes. Kokabel taught the signs of the stars. Ziqel taught the signs of the shooting stars. Arteqoph taught the signs of the earth. Shamsiel taught the signs of the sun. Sahriel taught the signs of the moon. And they all began to reveal mysteries to their wives and to their children. 4 (And) as men were perishing, the cry went up to heaven.”
1 Enoch 9:1, 6-10 1 Then Michael and Sariel and Raphael and Gabriel looked down from the sanctuary of heaven upon the earth and saw much bloodshed upon the earth. All the earth was filled with the godlessness and violence that had befallen it. … 6 You see what Asael has done, who has taught all iniquity upon the earth, and has revealed the eternal mysteries that are in heaven, which the sons of men were striving to learn. 7 And (what) Shemihazah (has done) to whom you gave authority to rule over them who are with him. 8 They have gone in to the daughters of the men of earth, and they have lain with them, and have defiled themselves with the women. And they have revealed to them all sins, and have taught them to make hate-producing charms. 9 And now behold, the daughters of men have born sons from them, giants, half-breeds. And the blood of men is shed upon the earth, And the whole earth is filled with iniquity. 10 And now behold, the spirits of the souls of the men who have died make suit; and their groan has come up to the gates of heaven; and it does not cease to come forth from before the iniquities that have come upon the earth. …
1 Enoch 10:1 1 Then the Most High said, and the Great Holy One spoke. And he sent Sariel to the son of Lamech [Noah], saying, 2 “Go to Noah and say to him in my name, ‘Hide yourself. And reveal to him that the end is coming, that the whole earth will perish; And tell him that a deluge is about to come on the whole earth and destroy everything on the earth. 3 Teach the righteous one what he should do, the son of Lamech how he may preserve himself alive and escape forever. From him a plant will be planted, and his seed will endure for all the generations of eternity.”
1 Enoch 10:4-8 4 To Raphael he said, “Go, Raphael, and bind Asael hand and foot; and cast him into the darkness; And make an opening in the wilderness that is in Doudael. 5 There cast him, and lay beneath him sharp stones and jagged stones. And cover him with darkness, and let him dwell there forever. Cover up his face, and let him not see the light. 6 And on the day of the great judgment, he will be led away to the burning conflagration. 7 And heal the earth, which the watchers have desolated; and announce the healing of the earth; that the plague may be healed, and all the sons of men may not perish because of the mystery that the watchers told and taught their sons. 8 And all the earth was made desolate by the deeds of the teaching of Asael. And over him write all the sins.”
1 Enoch 10:9-10 9 And to Gabriel he said, “Go, Gabriel, to the bastards; to the half-breeds, to the sons of miscegenation; and destroy the sons of the watchers from among the sons of men; send them against one another in a war of destruction. And length of days they will not have; 10 and no petition will be (granted) to their fathers in their behalf, that they should expect to live an eternal life, nor even that each of them should live five hundred years.”
1 Enoch 15:8-12, 16:1 8 But now the giants who were begotten by the spirits and flesh—they will call them evil spirits upon the earth, for their dwelling will be upon the earth. 9 The spirits that have gone forth from the body of their flesh are evil spirits, for from humans they came into being, and from the holy watchers was the origin of their creation. Evil spirits they will be on the earth, and evil spirits they will be called. 10 The spirits of heaven, in heaven is their dwelling; but the spirits begotten in the earth, on earth is their dwelling. 11 And the spirits of the giants <lead astray>, do violence, make desolate, and attack and wrestle and hurl upon the earth and <cause illnesses>. They eat nothing, but abstain from food and are thirsty and smite. 12 These spirits (will) rise up against the sons of men and against the women, for they have come forth from them. Chapter 16:1 1 “From the day of the slaughter and destruction and death of the giants, from the soul of whose flesh the spirits are proceeding, they are making desolate without (incurring) judgment. Thus they will make desolate until the day of the consummation of the great judgment, when the great age will be consummated. It will be consummated all at once.
1 Enoch 10:11-15 11 And to Michael he said, “Go, Michael, bind Shemihazah and the others with him, who have united themselves with the daughters of men, so that they were defiled by them in their uncleanness. 12 And when their sons perish and they see the destruction of their beloved ones, bind them for seventy generations in the valleys of the earth, until the day of their judgment and consummation, until the eternal judgment is consummated. 13 Then they will be led away to the fiery abyss, and to the torture, and to the prison where they will be confined forever. 14 And everyone who is condemned and destroyed henceforth will be bound together with them until the consummation of their generation. <And at the time of the judgment, which I shall judge, they will perish for all generations.> 15 Destroy all the spirits of the half-breeds and the sons of the watchers, because they have wronged men.
1 Enoch 12:3-6 3 I, Enoch, was standing, blessing the Lord of majesty, the King of the ages. And behold, the watchers of the Great Holy One called me, Enoch the scribe, and said to me, 4 “Enoch, righteous scribe, go and say to the watchers of heaven—who forsook the highest heaven, the sanctuary of the(ir) eternal station, and defiled themselves with women. As the sons of earth do, so they did and took wives for themselves. 5 And they wrought great desolation on the earth—‘You will have no peace or forgiveness.’ 6 “And concerning their sons, in whom they rejoice—The slaughter of their beloved ones they will see; and over the destruction of their sons they will lament and make petition forever, and they will have no mercy or peace.
1 Enoch 13:4-7 4 And they asked that I write a memorandum of petition for them, that they might have forgiveness, and that I recite the memorandum of petition for them in the presence of the Lord of heaven. 5 For they were no longer able to speak or to lift their eyes to heaven out of shame for the deeds through which they had sinned and for which they had been condemned. 6 Then I wrote out the memorandum of their petition, and the requests concerning themselves, with regard to their deeds individually, and concerning <their sons> for whom they were making request, that they might have forgiveness and longevity. 7 And I went and sat by the waters of Dan in the land of Dan, which is south of Hermon, to the west. I recited (to God) the memorandum of their petition until I fell asleep.
1 Enoch 14:4-6 4 I wrote up your petition, and in the vision it was shown to me thus, that you will not obtain your petition for all the days of eternity; but judgment has been consummated in the decree against you, 5 that from now on you will not ascend into heaven for all the ages; and it has been decreed to bind you in bonds in the earth for all the days of eternity. 6 And that before these things, you will see the destruction of your sons, your beloved ones; And that you will have no pleasure in them; but they will fall before you by the sword.
1 Enoch 15:1-7 1 But he answered and said to me—and I heard his voice—“Fear not, Enoch, righteous man and scribe of truth. Come here, and hear my voice. 2 Go and say to the watchers of heaven, who sent you to petition in their behalf, ‘You should petition in behalf of men, and not men in behalf of you. 3 Why have you forsaken the high heaven, the eternal sanctuary; and lain with women, and defiled yourselves with the daughters of men; and taken for yourselves wives, and done as the sons of earth; and begotten for yourselves sons; giants? 4 You were holy ones and spirits, living forever. With the blood of women you have defiled yourselves, and with the blood of flesh you have begotten; And with the blood of men you have lusted, and you have done as they do—flesh and blood, who die and perish. 5 Therefore I gave them women, that they might cast seed into them, and thus beget children by them, that nothing fail them upon the earth. 6 But you originally existed as spirits, living forever, and not dying for all the generations of eternity. 7 Therefore I did not make women among you.’ The spirits of heaven, in heaven is their dwelling;
" And now, the giants, who are produced from the spirits and flesh, shall be called evil spirits upon the earth, and on the earth shall be their dwelling. Evil spirits have proceeded from their bodies; because they are born from men and from the holy Watchers is their beginning and primal origin; they shall be evil spirits on earth, and evil spirits shall they be called. [As for the spirits of heaven, in heaven shall be their dwelling, but as for the spirits of the earth which were born upon the earth, on the earth shall be their dwelling.] And the spirits of the giants afflict, oppress, destroy, attack, do battle, and work destruction on the earth, and cause trouble: they take no food, but nevertheless hunger and thirst, and cause offenses. And these spirits shall rise up against the children of men and against the women, because they have proceeded from them.” – Enoch 15:8-12
Cf. Enoch 106:3-4
'(Howbeit,) I, Enoch, [have words of reassurance for you. This child [i.e., Noah] does not come from any of] the heavenly beings but from [your] son, Lamech.
"...And I, Enoch, answered and said unto him:
'The Lord will do a new thing on the earth, and this I have
already seen in a vision, and make known to thee that in the generation of
my father Jared some of the angels of heaven transgressed the word of the
Lord. And behold they commit sin and transgress the law, and have united
themselves with women and commit sin with them, and have married some
of them, and have begot children by them. And they shall produce on the
earth giants not according to the spirit, but according to the flesh, and there
shall be a great punishment on the earth, and the earth shall be cleansed
from all impurity. Yea, there shall come a great destruction over the whole
earth, and there shall be a deluge and 16 a great destruction for one year.
And this son who has been born unto you shall be left on the earth, and his
three children shall be saved with him: when all mankind that are on the
earth 8 shall die [he and his sons shall be saved]. And now make known to
thy son Lamech that he who has been born is in truth his son, and call his
name Noah; for he shall be left to you, and he and his sons shall be saved
from the destruction, which shall come upon the earth on account of all the
sin and all the unrighteousness, which shall be consummated on the earth
in his days. And after that there shall be still more unrighteousness than
that which was first consummated on the earth; for I know the mysteries of
the holy ones; for He, the Lord, has showed me and informed me, and I
have read (them) in the heavenly tablets." --Enoch 106:12-16
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