Genesis Chapter 6, verses 1 – 4
“Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the Sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. […] The Nephilim were on the earth in those days, and also afterward, when the Sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.”
The Nephilim are usually taken to be the children of the Sons of God and their human wives, although I notice that this passage doesn’t literally say so. They’re mentioned only once more in the Bible: in Numbers 13, the spies which Moses had sent into Canaan reported back that the land was inhabited by the Sons of Anak, or Anakites, who were descended from the Nephilim.
But there’s a lot of material that didn’t make it into the official Bible. The Book of Enoch is accepted only by the ancient Ethiopian Church, although it’s referenced in the Epistle of Jude, which surely makes it at least semi-official to the majority of Christians. In it, the fathers of the Nephilim are the Watchers, or Grigori, who are Angels. There are 200 of them on a mission to Earth, and they took a fancy to the ‘daughters of men’:
“And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. And the Angels, the children of heaven, saw and lusted after them, and said to one another: ‘Come, let us choose us wives from among the children of men and beget us children.'”
“And they became pregnant, and they bare great giants, whose height was three thousand ells who consumed all the acquisitions of men. And when men could no longer sustain them, the giants turned against them and devoured mankind. And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another’s flesh, and drink the blood.”
Not so good. However, in the mean time, the Grigori were teaching mankind all kinds of useful skills.
“And Azazel taught men to make swords, and knives, and shields, and breastplates, and made known to them the metals of the earth and the art of working them, and bracelets, and ornaments, and the use of antimony, and the beautifying of the eyelids, and all kinds of costly stones, and all colouring tinctures. And there arose much godlessness, and they committed fornication, and they were led astray, and became corrupt in all their ways. Semjaza taught enchantments, and root-cuttings, Armaros the resolving of enchantments, Baraqijal, taught astrology, Kokabel the constellations, Ezeqeel the knowledge of the clouds, Araqiel the signs of the earth, Shamsiel the signs of the sun, and Sariel the course of the moon.”
Note the fornication sentence embedded in the middle. I assume that the writer was thinking about the jewellery and makeup skills taught by Azazel: pure sinfulness.
The remainder of this section of the Book of Enoch has two slightly different stories about the eventual fate of the Nephilim and their fathers. At one point, God tells some henchmen (Archangels) to bind the Grigori and force them to watch their children killing each other all off, and then to cast them into eternal hellfire. However, in another passage, it’s said that God’s main purpose for the Great Flood was to eradicate the Nephilim, although this can’t have worked if their descendants, the Anakites, were still around at the time of Moses.
Incidentally, a height of three thousand ells is somewhere between two and three kilometres, depending on the exact definition of an ell (equivalent to a cubit).