Once upon a time, the fallen angels mated with humankind.
And it came to pass, when HaAdam began to multiply on the face of ha’adamah, and banot were born unto them, That the bnei HaElohim saw the banot HaAdam that they were tovot; and they took them nashim of all which they chose (Genesis 6:1-2 OJB).
The King James translates this as: When the human race began to increase, with more and more daughters being born, the sons of God noticed that the daughters of men were beautiful. They looked them over and picked out wives for themselves.
Out of this unholy union (Genesis 6:1-4) were born the Nephilim. This was back in the days (and also later) when there were giants in the land. The giants (Nephilim) came from the union of the sons of God and the daughters of men. These were the mighty men of ancient lore, the famous ones (Genesis 6:4).
These Nephilim, upon death, became the demons and evil spirits of today. In the New Testament, they are called “demons,” “unclean spirits,” and “evil spirits.”
Let not the dead live, let not the giants rise again… (Isaiah 26:14, Douay-Rheims Version).
Giants are the “demons.” Technically, they became demons. While they were alive, the Nephilim were giants, but only after they died did their spirits become demonic entities. You would think by the rarity of mention that demons must not exist. The Book of Enoch (a historical book, not Scripture) as well as the Bible tells us what became of these departed spirits of the Nephilim. The Nephilim became what we know now as “demons.” This is seen in Leviticus 17:7, Deuteronomy 32:17, 2 Chronicles 11:15, Psalms 106:37, and Job 26.
The Rephaim are formed, beneath the waters, also their inhabitants. The dead are in deep anguish, those beneath the waters and all that live in them (Job 26:5).
Noah’s flood destroyed their bodies, but now they are disembodied spirits looking for bodies through which they might once again affect their evil ways.
And now, the giants, who are produced from the spirits and flesh, shall be called evil spirits upon the earth, and on the earth shall be their dwelling. Evil spirits have proceeded from their bodies; because they are born from men and from the holy (Enoch 15:9 Historical Reference).
Fathers of the Third and Fourth Centuries: Lactantius, Venantius, Asterius, Victorinus, Dionysius, Apostolic Teaching and Constitutions:
“When, therefore, the number of men had begun to increase, God in His forethought, lest the devil, to whom from the beginning He had given power over the earth, should by his subtilty either corrupt or destroy men, as he had done at first, sent angels for the protection and improvement of the human race; and inasmuch as He had given these a free will, He enjoined them above all things not to defile themselves with contamination from the earth, and thus lose the dignity of their heavenly nature substantiæ, ‘essence.’ He plainly prohibited them from doing that which He knew that they would do, that they might entertain no hope of pardon. Therefore, while they abode among men, that most deceitful ruler (see 2 Cor. iv. 4), “the god of this world” of the earth, by his very association, gradually enticed them to vices, and polluted them by intercourse with women. Then, not being admitted into heaven on account of the sins into which they had plunged themselves, they fell to the earth. Thus from angels the devil makes them to become his satellites and attendants. But they who were born from these, because they were neither angels nor men, but bearing a kind of mixed Middle nature, were not admitted into hell, as their fathers were not into heaven. Thus there came to be two kinds of demons; one of heaven, the other of the earth. The latter are the wicked unclean spirits, the authors of all the evils which are done, and the same devil is their prince. Whence Trismegistus calls him the ruler of the demons. But grammarians say that they are called demons, as though dœmones. Plato approves of the etymology given by Lactantius; for he says that good men, distinguished by great honours, after their death became demons, in accordance with this title of prudence and wisdom. [See the whole subject in Lewis’ Plato, etc., p. 347.] That is, skilled and acquainted with matters: for they think that these are gods. They are acquainted, indeed, with many future events, but not all, since it is not permitted them entirely to know the counsel of God; and therefore they are accustomed to accommodate, to combine, qualify, or temperate their answers to ambiguous results. The poets both know them to be demons, and so describe them. Hesiod thus speaks, ‘These are the demons according to the will of Zeus, Good, living on the earth, the guardians of mortal men.’ And this is said for this purpose, because God had sent them as guardians to the human race; but they themselves also, though they are the destroyers of men, yet wish themselves to appear as their guardians, that they themselves may be worshipped, and God may not be worshipped. The philosophers also discuss the subject of these beings. For Plato attempted even to explain their natures in his ‘Banquet;’ and Socrates said that there was a demon continually about him, who had become attached to him when a boy, by whose will and direction his life was guided. The art also and power of the Magi altogether consists in the influences and Aspirations of these; invoked by whom they deceive the sight of men with deceptive illusions, Blinding tricks, juggleries so that they do not see those things which exist, and think that they see those things which do not exist. These contaminated and abandoned spirits, as I say, wander over the whole earth, and contrive a solace for their own perdition by the destruction of men. Therefore they fill every place with snares, deceits, frauds, and errors; for they cling to individuals, and occupy whole houses from door to door, and assume to themselves the name of genii; for by this word they translate demons in the Latin language. They consecrate these in their houses, to these they daily pour out. They lavish. The word implies profuse and excessive libations of wine, and worship; the wise demons as gods of the earth, and as averters of those evils which they themselves cause and impose. And these, since spirits are without substance. Thin, unsubstantial, as opposed to corporeal. The ancients inclined to the opinion that angels had a body, not like that of man, but of a slight and more subtle nature. Probably Lactantius refers to this idea in using the word tenuis. How opposed this view is to Scripture is manifest. [Not so manifest as our translator supposes. I do not assert what Lactantius says to be scriptural correct: but it certainly is not opposed to many facts as Scripture states them; whether figuratively or otherwise, I do not venture a suggestion.] and not to be grasped, insinuate themselves into the bodies of men; and secretly working in their inward parts, they corrupt the health, hasten diseases, terrify their souls with dreams, harass their minds with phrenzies, that by these evils they may compel men to have recourse to their aid.”