The Book of Enoch, which was found in the Dead Sea, tells a story that goes into the details of what Genesis 6 just mentions very briefly. That the Fallen Angels, The Watchers, procreated giants on Earth when they mixed with the daughters of the men. These creatures were known as the Rephaim amongst other names. They did not have a soul as we, humans have, so when they died their spirits turned into Demons. These are the wars that Paul warned us about in Ephesians 6:12 : "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places."
BOOK OF ENOCH - CHAPTER VI.
- And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. 2. 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.' 3. 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.' 4. 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.' 5. Then sware they all together and bound themselves by mutual imprecations upon it. 6. 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. 7. 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. 8. These are their chiefs of tens.
CHAPTER VII.
- 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. 2. And they became pregnant, and they bare great giants, whose height was three thousand ells: 3. Who consumed all the acquisitions of men. And when men could no longer sustain them, 4. the giants turned against them and devoured mankind. 5. And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another's flesh, and drink the blood. 6. Then the earth laid accusation against the lawless ones.
This two chapters of this Book of Enoch is aligned with what the Bible states in Genesis 6 :
"When human beings began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. 3 Then the LORD said, “My Spirit will not contend with[a] humans forever, for they are mortal[b]; their days will be a hundred and twenty years.”
4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown. 5 The LORD saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. 6 The LORD regretted that he had made human beings on the earth, and his heart was deeply troubled. 7 So the LORD said, “I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them.” 8 But Noah found favor in the eyes of the LORD."