Bible has many places that contain the difficult verses for interpretation and understanding. One of them is recorded in Genesis 6, where the account of how the "nephilim" came into the ancient world and why God caused the great Flood to wipe them off the surface of the ground including all creatures that He had made. Etemologically to say, the word "nephilim" came from Hebrew verb 'napal' which means 'to fall', so 'nephilim' can be rendered as 'fallen ones'. This could mean 'fallen ones' from the moral or ontological standpoint.
They were physically mighty ones and men of fame in pre-Deluge generation. They were all believed to have been perished in the Noah's Flood and no further record for them is found in the Old Testament. Only the history of Noah's sons and their children is recorded thereafter. Who were 'nephilim'? Genesis 6 tells that they were born between 'the sons of God' and 'the daughters of men'. It's not that difficult to know who were 'the daughters of men' but not 'sons of God'. There have been many interpretatons on this verse throughout the Christian history. The most acceptable understanding is that they are 'the children of Seth', who called the name of Jehovah for the first time(Gen. 5:26).
This interpretation seems to fit well to the entire Biblical idea, yet leaves a further discussion about why they turn out to be fallen ones who deserved the God's judgment? And what is more,if the blood lines of Seth were mixed with that of daughters of men, why should this be the ground upon which God's judgment must have fallen?
Genesis 6 testifies that God felt hurt at His heart and felt regrets that He had made them(man and other animals)and so He decided to wipe them off the surface of the ground. Then who were they - nephilim?
I must say this very carefully after many years of investigation and meditation on the Word of God. In my understanding, they were the children born between the sons of God(angelic beings) and the daughters of men(human female). So to speak, a hybrid kind of human species whose inclination of the thoughts of heart is evil all the time(Gen. 6:5).
This position has not been accepted by many because the Bible says that angels do not marry and they have no physical ability to multiply. It's very true. But when the Lord Jesus said, 'angels do not marry' He meant by this that in heaven there is no institution of marriage established to multiply. Genesis 6 account tells us that there must have been an unusual copulation between the sons of God(angelic beings) and the daughters of men(human female) by way of unknown method to us.
Jude 6 tells us that the angels who did not keep their original position but forsook their own proper dwelling place God has reserved with eternal bonds under dense darkness for the Judgment of the Great Day. They were believed to be these angels who came to earth and took the daughters of men as wives for themselves, namely all whom they chose. God never told Adam to take as many wives as he choose. But the fallen angels did against God's will and they will be punished accordingly.
I can not go further because I do not know more than this on this specific topic. If I have more understanding, I can write more on this.
Have a nice day, you folks!