The Sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
Genesis 6:2

Some commentators believe that the expression 'sons of God' refers to the 'godly line' of Seth, and 'daughters of men' to women from the line of Cain.
Commentary on the Living Bible

The Book of Giants was another literary work concerned with Enoch, widely read (after translation into the appropriate languages) in the Roman empire....The 'giants' were believed to be the offspring of fallen angels (the Nephilim; also called Watchers) and human women.
Robert Eisman and Michael Wise, The Dead Sea Scrolls Uncovered

In The Book of Giants (i.e., 4Q531, 6Q8 Frag. 2 and 4Q530 Col. 2), the name of one of the giants is Gilgamesh, the Babylonian hero and subject of a great epic written in the third millennium B.C.E.
Michael Wise, Martin Abegg, Jr., and Edward Cook, The Dead Sea Scrolls: A New Translation (1996) p. 247)

The root of Nephilim is nephel which means untimely birth, abortion and miscarriage.

The Nefilim were upon the Earth in those days and thereafter too. Those sons of the gods who cohabited with the daughters of the Adam, and they bore children into them. They were the Mighty Ones of Eternity, the People of the Shem.
Genesis 6:4

The tradition in Genesis 6.4 may reflect the Canaanite myth of the birth of minor gods from the union of El and human women. The conception of the Rephaim as supermen may reflect the Canaanite tradition of defunct kings as rp'um, or Dispensers of fertility. The identity in tradition of 'the fallen ones' of Genesis 6:4 and the Rephaim is supported by the nature of the latter in Proverbs 2:18; Job 26:5 and Phoenician funerary inscriptions.
John Gray, Near Eastern Mythology

Megalithic monuments, found by the Hebrews on their arrival in Canaan, will have encouraged legends about giants; as in Greece, where the monstrous man-eating Cyclopes were said by story-tellers ignorant of ramps, levers and other Mycenaean engineering devices, to have lifted single- handed the huge blocks of stone that form the walls of Tiryns, Mycenae and other ancient cities.
Robert Graves and Raphael Patai, Hebrew Myths: The Book of Genesis

On a parchment fragment 4Q201(En ara) copied ca. 200-150 B.C.E. found at Qumrum: 13. [They (the leaders) and all ... of them took for themselves] wives from all that they chose and [they began to cohabit with them and to defile themselves with them]; and to teach them sorcery and [spells and the cutting of roots; and to acquaint them with herbs.] And they become pregnant by them and bo[re (great) giants three thousand cubits high ...]
Book of Enoch (from Translation by J. C. Greenfield)

Later Jewish tradition has it that their seduction was at least partly their own fault since they had taught the girls the art of cosmetics, and so had begun the awful progress of mankind to degeneracy and sexual abandon. More important, they taught them charms and enchantments, the cutting of roots, and make them acquainted with plants... (Enoch 7:1ff).
John M. Allegro, The Sacred Mushroom and the Cross

The Hebrew word for giants (nephilum) literally means the fallen-down-ones because these tall celestial beings fell from the sky. Their half-breed progeny and their descendants are often mentioned in the early books of the Old Testament until the last of them were finally killed off. They were known as the Rephaim [Hebrew for 'phantoms'], Emim, Anakim, Horim, Avim, and Zamzummim. Some scholars speculate that this tradition of giants born from the union of gods and humans formed the basis for the demigod of Greek mythology.
Raymond E. Fowler, The Watchers

Those giants...are termed n'philim (lit. 'those who have fallen' or 'perished'). A similar tradition mentions such a race of primordial giants in the Rephaim.
John Gray, Near Eastern Mythology

The Nefilim ('Fallen Ones') bore many other tribal names, such as Emim ('Terrors'), Repha'im ('Weakeners'), Gibborim ('Giant Heroes'), Zamzummim ('Achievers'), Anakim ('Long-necked' or 'Wearers of Necklaces'), Awwim ('Devastators' or 'Serpents'). One of the Nefilim named Arba is said to have built the city of Hebron, called 'Kiriath-Arba' after him, and become the father of Anak whose three sons, Sheshai, Ahiman and Talmai, were later expelled by Joshua's comrade Caleb. Since, however, arba means 'four' in Hebrew, Kiriath-Arba may have originally have meant 'City of Four,' a reference to its four quarters mythically connected with the Anakite clans: Anak himself and his 'sons' Sheshai, Ahiman and Talmai.
Robert Graves and Raphael Patai, Hebrew Myths: The Book of Genesis

And there we saw the Nephilim, the sons of Anak, which come of the Nephilim: and we were in our own sight as grasshoppers, and so we were in their sight.
Numbers 13:33

The Emim - a large and numerous people, as tall as the Anakim - had formerly inhabited it [Moab]. Like the Anakim, they are usually reckoned as Rephaim, though the Moabites call them Emim.
Now only King Og of Bashan was left of the remnant of the Rephaim. In fact his bed, an iron bed, can still be seen in Rabbah of the Ammonites. By the common cubit [63.5 cm/25 in] it is nine cubits [5.7 m/18.75 ft] long and four cubits wide.
Deuteronomy 2:11, 3:11

Our skills and behavior are finely attuned to our size. We could not be twice as tall as we are, for the kinetic energy of a fall would then be 16 to 32 times as great, and our sheer weight (increased eightfold) would be more than our legs could support. Human giants of eight to nine feet have either died young or been crippled early by failure of joints and bones. At half our size, we could not wield a club with sufficient force to hunt large animals (for kinetic energy would decrease 16 to 32-fold); we could not impart sufficient momentum to spears and arrows; we could not cut or split wood with primitive tools or mine minerals with picks and chisels. Since these all were essential activities in our historical development, we must conclude that the path of our evolution could only have been followed by a creature very close to our size. I do not argue that we inhabit the best of all possible worlds, only that our size has limited our activities and, to a great extent, shaped our evolution.
Stephen Jay Gould, Sizing Up Human Intelligence, Physical Anthropology 96/97 pp.150-51

For her house leads down to death and her paths to the spirits of the dead.
Proverbs 2:18

The dead are in deep anguish, those beneath the waters and all that live in them.
Job 26:5

He that hath the Son hath life; and he that hath not the Son of God hath not life.
I John 5:12

Who were the Nephilim?

(Heb. nephi„li„m; Numbers 13:13). The form of the Heb. word denotes a plural verbal adjective or noun of passive signification, certainly from na„pal, “to fall,” so that the connotation is “the fallen ones,” clearly meaning the unnatural offspring that were on the earth in the years before the Flood, “and also afterward, when the sons of God came in to the daughters of men, and they bore children to them” (Genesis 6:4). The mention of the great stature of the Nephilim, the sons of Anak, in the evil report that the ten spies brought of the land of Canaan (Numbers 13:33) together with the LXX rendering, gigantes, suggested the translation giants. They were exceedingly wicked and violent so that “every intent” of the thoughts of men’s hearts “was only evil continually” (Genesis 6:5).

(Genesis 6:4; Numbers 13:33). The Nephilim are considered by many to be giant demigods, the unnatural offspring of the “daughters of men” (mortal women) in cohabitation with the “sons of God” (angels; cf. Genesis 6:1-4). This utterly unnatural union, violating God’s created order of being, was such a shocking abnormality as to necessitate the worldwide judgment of the Flood. Another view of the Nephilim is that they were particularly violent (the name is from a root, “to fall,” i.e., on other people), strong (“mighty”), and infamous (“men of renown”) people who predated the marriages of v. 2. This viewpoint often explains the unions as intermarriage of the godly line of Seth (described in 4:25–5:32) with the ungodly line of Cain (4:1-24).
From the book The Bible Has the Answer, by Henry M. Morris and Martin E. Clark.

Assyrian and Akkadian Demonology

Iranian Demonology

Jewish Demonology

Early Christian Demonology

Medieval to Modern Demonology

The Book of Enoch

The Nephilim