Mothers of the Nephilim
In antediluvian times, "sons of God" saw the beauty of the "daughters of men" and married and had intercourse with them, producing the Nephilim, often translated as "giants" or "fallen ones".
Now it happened, 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 desirable; and they took wives for themselves, whomever they chose and desired. Then the Lord said,My Spirit shall not strive and remain with man forever, because he is indeed flesh [sinful, corrupt—given over to sensual appetites]; nevertheless his days shall yet be a hundred and twenty years.There were Nephilim (men of stature, notorious men) on the earth in those days—and also afterward—when the sons of God lived with the daughters of men, and they gave birth to their children. These were the mighty men who were of old, men of renown (great reputation, fame).— Genesis 6:1–6:4 (AMP)
Enoch and Jubilees account
According to 1 Enoch and Jubilees, the "sons of God" mentioned in Genesis were angels known as Grigori, or Watchers. Rebellious Grigori agreed to share the repercussions for their actions and had intercourse with human women. The women gave birth to giants, and the giants ate humans and were a bright on the Earth.
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 the 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 Semjâzâ, who was their leader, said unto them:I fear ye will not indeed agree to do this deed, and I alone shall have to pay the penalty of a great sin.And they all answered him and said:Let us all swear an oath, and all bind ourselves by mutual imprecations not to abandon this plan but to do this thing.Then sware they all together and bound themselves by mutual imprecations upon it. And they were in all two hundred; who descended ⌈in the days⌉ of Jared on the summit of Mount Hermon, and they called it Mount Hermon, because they had sworn and bound themselves by mutual imprecations upon it. And these are the names of their leaders: Sêmîazâz, their leader, Arâkîba, Râmêêl, Kôkabîêl, Tâmîêl, Râmîêl, Dânêl, Êzêqêêl, Barâqîjâl, Asâêl, Armârôs, Batârêl, Anânêl, Zaqîêl, Samsâpêêl, Satarêl, Tûrêl, Jômjâêl, Sariêl. These are their chiefs of tens.
And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants. 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. Then the earth laid accusation against the lawless ones.— 1 Enoch 6:1–7:6 (R. H. Charles)
And it came to pass when the children of men began to multiply on the face of the earth and daughters were born unto them, that the angels of God saw them on a certain year of this jubilee, that they were beautiful to look upon; and they took themselves wives of all whom they chose, and they bare unto them sons and they were giants. And lawlessness increased on the earth and all flesh corrupted its way, alike men and cattle and beasts and birds and everything that walketh on the earth-all of them corrupted their ways and their orders, and they began to devour each other, and lawlessness increased on the earth and every imagination of the thoughts of all men (was) thus evil continually.— Jubilees 5:1–5:2 (R. H. Charles)
God knew of the wickedness that was occurring and heard the cries of those killed by the Nephilim. God then had His angels bind the rebellious Grigori and had had His angels have the Nephilim kill each other, and God caused the Flood in order to rid the world of evil, leaving only the virtuous Noah and his progeny to repopulate the Earth.
The dead Nephilim produced evil spirits, or demons, that tempt people after the Flood.