“Who / what were the Nephilim?”……The Nephilim (“fallen ones, giants”) were the offspring of sexual relationships between the sons of God and daughters of men

Genesis 6:1-4. Then the people began to multiply on the earth, and daughters were born to them. The sons of God saw the beautiful women and took any they wanted as their wives.3 Then the Lord said, “My Spirit will not put up with humans for such a long time, for they are only mortal flesh. In the future, their normal lifespan will be no more than 120 years.” In those days, and for some time after, giant Nephilites lived on the earth, for whenever the sons of God had intercourse with women, they gave birth to children who became the heroes and famous warriors of ancient times.

There is much debate as to the identity of the “sons of God.” It is our contention that the “sons of God” were fallen angels (demons) who mated with human females and/or possessed human males and then mated with human females. These unions resulted in offspring, the Nephilim, that were “heroes of old, men of renown” (Genesis 6:4).……Why would the demons do such a thing? The Bible does not specifically give us the answer. The demons are evil, twisted beings—so nothing they do should surprise us. As to a distinct motivation, the best speculation is that the demons were attempting to pollute the human blood line in order to prevent the coming of the Messiah, Jesus Christ. God had promised that a Messiah would come from the line of Eve () who would crush the head of the serpent, Satan.

Genesis 3:15And I will cause hostility between you and the woman, and between your offspring and her offspring.He will strike your head, and you will strike his heel.”

So, the demons were possibly attempting to prevent this by polluting the human bloodline, making it impossible for a sinless Messiah to one day be born. Again, this is not a specifically biblical answer, but it is plausible and not in contradiction to anything the Bible teaches.……What were the Nephilim? According to Hebraic and other legends (the Book of Enoch and other non-biblical writings), they were a race of giants and super-heroes who did acts of great evil. Their great size and power likely came from the mixture of demonic “DNA” with human genetics. All that the Bible directly says about them is that they were “heroes of old, men of renown” (Genesis 6:4). The Nephilim were not aliens, they were literal, physical beings produced from the union of the sons of God and daughters of men (Genesis 6:1-4).……What happened to the Nephilim? The Nephilim were one of the primary reasons for the great flood in Noah’s time. Immediately after the Nephilim are mentioned, God’s Word tells us this:

Genesis 6:5-7“The Lord observed the extent of human wickedness on the earth, and he saw that everything they thought or imagined was consistently and totally evil.So the Lord was sorry he had ever made them and put them on the earth. It broke his heart. And the Lord said, “I will wipe this human race I have created from the face of the earth. Yes, and I will destroy every living thing—all the people, the large animals, the small animals that scurry along the ground, and even the birds of the sky. I am sorry I ever made them.”.

So, God proceeded to flood the entire earth, killing everyone and everything (including the Nephilim) other than Noah and his family and the animals on the ark

Genesis 6:11-22.Now God saw that the earth had become corrupt and was filled with violence. God observed all this corruption in the world, for everyone on earth was corrupt. So God said to Noah, “I have decided to destroy all living creatures, for they have filled the earth with violence. Yes, I will wipe them all out along with the earth!

“Build a large boat from cypress wood and waterproof it with tar, inside and out. Then construct decks and stalls throughout its interior. Make the boat 450 feet long, 75 feet wide, and 45 feet high. Leave an 18-inch opening below the roof all the way around the boat. Put the door on the side, and build three decks inside the boat—lower, middle, and upper. “Look! I am about to cover the earth with a flood that will destroy every living thing that breathes. Everything on earth will die. But I will confirm my covenant with you. So enter the boat—you and your wife and your sons and their wives. Bring a pair of every kind of animal—a male and a female—into the boat with you to keep them alive during the flood. Pairs of every kind of bird, and every kind of animal, and every kind of small animal that scurries along the ground, will come to you to be kept alive. And be sure to take on board enough food for your family and for all the animals.” So Noah did everything exactly as God had commanded him.

Were there Nephilim after the flood?

Genesis 6:4 tells us,“In those days, and for some time after, giant Nephilites lived on the earth, for whenever the sons of God had intercourse with women, they gave birth to children who became the heroes and famous warriors of ancient times..”

It seems that the demons repeated their sin sometime after the flood as well. However, it likely took place to a much lesser extent than it did prior to the flood. When the Israelites spied out the land of Canaan, they reported back to Moses:

Numbers 13:33We even saw giants (Hebrew Nephilim) there, the descendants of Anak.(the descendants of Anak come from the Nephilim) Next to them we felt like grasshoppers, and that’s what they thought, too!”.

Now, this passage does not specifically say that the Nephilim were genuinely there, only that the spies thought they saw the Nephilim. It is more likely that the spies witnessed very large people in Canaan and mistakenly believed them to be the Nephilim. Or, it is possible that after the flood the demons again mated with human females, producing more Nephilim. Whatever the case, these “giants” were destroyed by the Israelites during their invasion of Canaan

Joshua 11:21-22During this period Joshua destroyed all the descendants of Anak, who lived in the hill country of Hebron, Debir, Anab, and the entire hill country of Judah and Israel. He killed them all and completely destroyed their towns.22 None of the descendants of Anak were left in all the land of Israel, though some still remained in Gaza, Gath, and Ashdod.

and later in their history

Deuteronomy 3:11King Og of Bashan was the last survivor of the giant Rephaites. His bed was made of iron and was more than thirteen feet long and six feet wide. It can still be seen in the Ammonite city of Rabbah.

1 Samuel 17:4Then Goliath, a Philistine champion from Gath, came out of the Philistine ranks to face the forces of Israel. He was over nine feet tall!

Hebrew 6 cubits and 1 span [which totals about 9.75 feet or 3 meters]; Dead Sea Scrolls and Greek version read 4 cubits and 1 span [which totals about 6.75 feet or 2 meters].

…What prevents the demons from producing more Nephilim today? It seems that God put an end to demons mating with humans by placing all the demons who committed such an act in the Abyss. Jude verse 6 tells us, “And the angels who did not keep their positions of authority but abandoned their own home—these he has kept in darkness, bound with everlasting chains for judgment on the great Day.” Obviously, not all of the demons are in “prison” today, so there must have been a group of demons who committed further grievous sin beyond the original fall. Presumably, the demons who mated with human females are the ones who are “bound with everlasting chains.” This would prevent any more demons from attempting such an act.