Demons are spirits that are malignant or evil in nature (indeed, the terms ‘demons’ and ‘evil spirits’ can, and are often, used interchangably).
The origin of demons is not explictly clear in the Scripture, and some people hold the view that demons are fallen angels, and use Revelation 12:3-4 as scriptural support, explaining that the “stars” referred to in this passage of scripture were fallen angels who followed Satan and became demons:
3 Then another sign appeared in heaven: an enormous red dragon with seven heads and ten horns and seven crowns on its heads. 4 Its tail swept a third of the stars out of the sky and flung them to the earth. The dragon stood in front of the woman who was about to give birth, so that it might devour her child the moment he was born.
On the other hand, others (and I would be included in this category) hold the view that demons are the disembodied spirits of the Nephilim.
Ok, I need to explain a couple of terms here. “Disembodied” means “bodiless” and “without body“. Christians believe that when a human being dies, life doesn’t just end there, but that the human soul gets transferred to either hell (Acts 2:27; in particular, see the Webster’s bible translation), or to be with the Lord Jesus Christ in paradise (Luke 23:43).
A similar principle applies for this species known as the Nephilim. When a Nephilim dies (or, more likely, got killed), that is not the end of its “life”. Rather, while the body of Nephilim undergoes decay, its spirit remains on earth, and is free to roam the earth.
Next, I need to explain who were the Nephilim. According to Genesis 6:4 (NLT), they were the offspring of the “sons of God” (which I believe refers to the fallen angels) and the “daughters of men” (which I believe refers to the human women) that were conceived before the Flood:
In those days, and for some time after, giant Nephilites lived on the earth, for whenever the sons of God had intercourse with women, they gave birth to children who became the heroes and famous warriors of ancient times.
I acknowledge that it is very controversial to suggest that fallen angels and human women can have sexual intercourse and produce offspring. Indeed, those who protest such a perspective would typically cite Matthew 22:30 to support their position:
At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven.
but in my perspective, this verse only supports the position of what it says (that is, that angels do not marry in heaven), but does not disprove that angels are not capable of having sexual intercourse with human beings, if not restrained from doing so.
While the bible may be rather silent about the connection between fallen angels, Nephilim and evil spirits, on the other hand, the Book of Enoch is very explicit it (Enoch 15:2b-12; emphasis mine):
And go, say to the Watchers of heaven, who have sent thee to intercede for them: “You should intercede” for men, and not men 3 for you: Wherefore have ye left the high, holy, and eternal heaven, and lain with women, and defiled yourselves with the daughters of men and taken to yourselves wives, and done like the children 4 of earth, and begotten giants (as your) sons? And though ye were holy, spiritual, living the eternal life, you have defiled yourselves with the blood of women, and have begotten (children) with the blood of flesh, and, as the children of men, have lusted after flesh and blood as those also do who die 5 and perish. Therefore have I given them wives also that they might impregnate them, and beget 6 children by them, that thus nothing might be wanting to them on earth. But you were formerly 7 spiritual, living the eternal life, and immortal for all generations of the world. And therefore I have not appointed wives for you; for as for the spiritual ones of the heaven, in heaven is their dwelling. 8 And now, the giants, who are produced from the spirits and flesh, shall be called evil spirits upon 9 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 Watchers is their beginning and primal origin; 10 they shall be evil spirits on earth, and evil spirits shall they be called. [As for the spirits of heaven, in heaven shall be their dwelling, but as for the spirits of the earth which were born upon the earth, on the earth shall be their dwelling.] And the spirits of the giants afflict, oppress, destroy, attack, do battle, and work destruction on the earth, and cause trouble: they take no food, but nevertheless 12 hunger and thirst, and cause offences. And these spirits shall rise up against the children of men and against the women, because they have proceeded from them.
I realise that some Christians may have reservations about using a non-biblical book (I’m refering to the Book of Enoch) to glean spiritual truths. Although the Book of Enoch is indeed not included in the bible, nevertheless, I think it is a valuable book. For instance, the book of Jude references the Book of Enoch (that is, the book of Jude has a description of an event that the author took from the book of Enoch; see verses 14-15). The Book of Enoch is also included in the bible of the Ethiopian Orthodox Tewahedo Church and Eritrean Orthodox Tewahedo Church.
So, in short, I’ve tried to explain here my view that ‘demons’ or ‘evil spirits’ (the terms are interchangable) are the spirits of Nephilim (also known as ‘Giants’) that have died, and that the Nephilim were the hybrid offspring of fallen angels and human women.