There are 2 theories about the Nephilim.
1- they are humans from the godly line of Seth.
2- they are fallen angels.

Unfortunately for me- both theories seem to have enough scripture support to make me want to believe both of them. I am not clear on this.

4 The Neph?i·lim (Or, “The Fellers.” Heb., han·Nephi·lim?, “those who cause others to fall down.”) proved to be in the earth in those days, and also after that, when the sons of the [true] God continued to have relations with the daughters of men and they bore sons to them, they were the mighty ones who were of old, the men of fame.

Bible commentators, considering verse 4, have offered several suggestions as to the identity of these Nephilim. Some have thought that the derivation of the name indicates that the Nephilim had fallen from heaven, that is, that they were ‘fallen angels’ who mated with women to produce “mighty ones . . . the men of fame.” Other scholars, focusing their attention particularly on the statement “and also after that” (vs 4), have said the Nephilim were not the ‘fallen angels’ or the “mighty ones,” since the Nephilim “proved to be in the earth in those days” before the sons of God had relations with women. These latter scholars hold the opinion that the Nephilim were simply wicked men like Cain—robbers, bullies, and tyrants who roamed the earth until they were destroyed by the Flood. Still another group, taking into consideration the context of verse 4, conclude that the Nephilim were not themselves angels, but were the hybrid offspring resulting from materialized angels having intercourse with the daughters of men.

Same as “gib·bo·rim?.” Certain Bible translations adjust the location of the phrase “and also after that,” placing it near the beginning of verse 4, thus identifying the Nephilim with the “mighty ones,” the gib·bo·rim?, mentioned in the latter part of the verse. For example: “In those days, as well as afterward, there were giants [Heb., han·nephi·lim?] on the earth, who were born to the sons of the gods whenever they had intercourse with the daughters of men; these were the heroes [Heb., hag·gib·bo·rim?] who were men of note in days of old.”—Ge 6:4, AT; see also Mo, NIV, and TEV.

The Greek Septuagint also suggests that both the “Nephilim” and “mighty ones” are identical by using the same word gi?gan·tes (giants) to translate both expressions.

Reviewing the account, we see that verses 1 to 3 tell of “the sons of the true God” taking wives and of Jehovah’s statement that he was going to end his patience with men after 120 years. Verse 4 then speaks of the Nephilim proving to be in the earth “in those days,” evidently the days when Jehovah made the statement. Then it shows that this situation continued “after that, when the sons of the true God continued to have relations with the daughters of men,” and describes in more detail the results of the union of “the sons of the true God” with women.

Who were the ‘sons of God’ that fathered the Nephilim?

Who were “the sons of the true God” that were involved? Were they men who were worshipers of Jehovah (as distinguished from the general run of wicked mankind), as some claim? Evidently not. The Bible implies that their marriage to the daughters of men resulted in whipping up the badness in the earth. Noah and his three sons, along with their wives, were the only ones in God’s favor and were the only ones preserved through the Deluge.—Ge 6:9; 8:15, 16; 1Pe 3:20.

Hence, if these “sons of the true God” were merely men, the question arises, Why were their offspring “men of fame” more than those of the wicked, or of faithful Noah? Also, the question might be asked, Why mention their marriage to the daughters of men as something special? Marriage and childbearing had been taking place for more than 1,500 years.

The sons of God mentioned at Genesis 6:2, therefore, must have been angels, spirit “sons of God.” This expression is applied to angels at Job 1:6; 38:7. This view is supported by Peter, who speaks of “the spirits in prison, who had once been disobedient when the patience of God was waiting in Noah’s days.” (1Pe 3:19, 20) Also Jude writes of “the angels that did not keep their original position but forsook their own proper dwelling place.” (Jude 6) Angels had the power to materialize in human form, and some angels did so to bring messages from God. (Ge 18:1, 2, 8, 20-22; 19:1-11; Jos 5:13-15) But heaven is the proper abode of spirit persons, and the angels there have positions of service under Jehovah. (Da 7:9, 10) To leave this abode to dwell on earth and to forsake their assigned service to have fleshly relations would be rebellion against God’s laws, and perversion.

The fame and dread of the Nephilim, it appears, gave rise to many mythologies of heathen people who, after the confusion of languages at Babel, were scattered throughout the earth. Though the historical forms of the Genesis account were greatly distorted and embellished, there was a remarkable resemblance in these ancient mythologies (those of the Greeks being only one example), in which gods and goddesses mated with humans to produce superhuman heroes and fearful demigods having god-man characteristics.

Edited for Fishy.

The darkness or Tartarus that Jude speaks of means their abilities were limited. From that time on, the demons access to humans were only by possession.

The Bible shows that the loyal angels could still form bodies to carry out the tasks given to them. The ones that stopped and had dinner with Abraham, and then went to save Lot, grabbing the family by the hand and pulling them out of the city.

These children we’re the Nephilim. Giants. People ask how fallen angels could get a women pregnant, well, they lost their divinity when they we’re cast out of heaven, I think. Regardless, the Bible say’s it so it’s true.

Greek myths may be originate from the lives of these “giants”…they we’re not like us. Even the two spies who reported back to Joshua stated that they we’re as “grasshoppers” compared to the giants in the land. Think about that one.