The Nephilim (aka "Grigori") were offspring of Angels who defied God, came to earth and married the daughters of men. Their offspring were giants, men of renown. Then evil became rampant on the earth and God destroyed mankind with the flood. ("sons of God" are Angels)
From the BIBLE
The term "Nephilim" occurs just twice in the Hebrew Bible, both in the Torah. The first is Genesis 6:1-4, immediately before the story of Noah's Ark:
1 When men began to increase in number on the earth and daughters were born to them,
2 The sons of God saw that the daughters of men were beautiful, and they married any of them they chose.
3 Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his 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 men and had children by them. They were the heroes of old, men of renown.
The second is Numbers 13:32-33, where the Hebrew spies report that they have seen fearsome giants in Canaan:
32 And they spread among the Israelites a bad report about the land they had explored. They said, "The land we explored devours those living in it. All the people we saw there are of great size.
33 We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them."
According to the Apocryphal Book of Enoch, the Grigori (also known as "Watchers"), are a group of fallen angels who mated with mortal women, giving rise to a race of giants known as the Nephilim.
From THE FORGOTTEN BOOKS OF EDEN in the "SECRETS OF ENOCH"
Chapter XVIII: Of the taking of Enoch to the fifth heaven
1 The men took me on to the fifth heaven and place me, and there I saw many and countless soldiers, called Grigori, of human appearance, and their size was greater than that of great giants and their faces withered, and the silence of their mouths perpetual, and there was no service on the fifth heaven, and I said to the men who were with me:
2 Wherefore are these very withered and their faces melancholy, and their mouths silent, and wherefore is there no service on this heaven?'
3 And they said to me: These are the Grigori, who with their prince Satanail rejected the Lord of light, and after them are those who are held in great darkness on the second heaven, and three of them went down on to earth from the Lord's throne, to the place Ermon, and broke through their vows on the shoulder of the hill Ermon and saw the daughters of men how good they are, and took to themselves wives, and befouled the earth with their deeds, who in all times of their age made lawlessness and mixing, and giants are born and marvellous big men and great enmity.
4 And therefore God judged them with great judgement, and they weep for their brethren and they will be punished on the Lord's great day.
STEVE QUAYLE on COAST TO COAST AM with George Noory
The Truth About Nephilim Giants
*DISCLAIMER: I don't believe mankind was genetically engineered by Extraterrestrials. I believe in the Biblical story of Adam and Eve.