Who/what is the seed of the serpent?
Gen 3:15 And I will put enmity between thee [the serpent] and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
Who/what were these ‘sons of God’ that took human women as wives and whose offspring were hybrid giants [Nephilim]? And after which the evil of humanity multiplied leading God to destroy [essentially] all flesh in the global flood [of Noah]:
Gen 6:1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
Gen 6:2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. […]
Gen 6:4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Gen 6:5 And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
Gen 6:6 And it repented the LORD that he had made man on the earth, and it grieved him at his heart.
Gen 6:7 And the LORD said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them.
Who/what are “they” which shall mingle their seed with man in the final chapter of human history?
Dan 2:43 And whereas thou sawest iron mixed with miry clay, they shall mingle themselves with the seed of men: but they shall not cleave one to another, even as iron is not mixed with clay.
Christ Himself alludes to this ‘coming full circle’ as we approach the times and just prior to His final return at armageddon:
Mat 24:37 But as the days of Noe were, so shall also the coming of the Son of man be.
Mat 24:38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark,
Mat 24:39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be.
All these questions, and many more, are addressed in the following article:
DO ALIEN/HUMAN HYBRIDS WALK AMONG US?
By Tom Horn
As discussed in Part 2 of this series, the manipulation of living tissue by ancient fallen angels led to superhuman hybridity made up of human and animal genetics known as Nephilim, an “Earth-born” facsimile or “fit extension” into which fallen angels incarnated. However, the long history of demonological phenomenon related to manipulation of biological matter suggests that versions of this curious activity have been ongoing ever since the Days of Noah. Today, what some call “alien abduction,” in which a breeding program allegedly exists resulting in alien/human hybrids, seems but a contemporary retelling of similar DNA harvesting and genetic manipulation by those mysterious beings whose activity was recorded throughout time. In his book, Confrontations—A Scientist’s Search for Alien Contact, highly regarded UFO researcher, Dr. Jacques F. Vallée, once phrased it this way: “Contact with [aliens is] only a modern extension of the age-old tradition of contact with nonhuman consciousness in the form of angels, demons, elves, and sylphs.” Later, Vallée more closely identified the operative power behind “aliens” as equivalent to the fallen angels in the Days of Noah:
Are these races only semi-human, so that in order to maintain contact with us, they need crossbreeding with men and women of our planet? Is this the origin of the many tales and legends where genetics plays a great role: the symbolism of the Virgin in occultism and religion, the fairy tales involving human midwives and changelings, the sexual overtones of the flying saucer reports, the biblical stories of intermarriage between the Lord’s angels, and terrestrial women, whose offspring were giants? 
Another highly respected and often quoted UFO researcher, John Keel, echoed the same when he stated in Operation Trojan Horse:
Demonology is not just another crackpot-ology. It is the ancient and scholarly study of the monsters and demons who have seemingly coexisted with man throughout history… The manifestations and occurrences described in this imposing literature are similar, if not entirely identical, to the UFO phenomenon itself. Victims of demonomania (possession) suffer the very same medical and emotional symptoms as the UFO contactees… The Devil and his demons can, according to the literature, manifest themselves in almost any form and can physically imitate anything from angels to horrifying monsters with glowing eyes. Strange objects and entities materialize and dematerialize in these stories, just as the UFOs and their splendid occupants appear and disappear, walk through walls, and perform other supernatural feats.