Origin of Demons

Book of Enoch

25 And though ye were holy, spiritual, living the eternal life, you have defiled
yourselves with the blood of women, and have begotten with the blood of flesh,
and, as the children of men, have lusted after flesh and blood as those also do
who die and perish.
26 Therefore have I given them wives also that they might impregnate them, and
beget children by them, that thus nothing might be wanting to them on earth.
27 But you were formerly 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.
28 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.
29 Evil spirits have proceeded from their bodies, because they are born from
men and from the Watchers is their beginning and primal origin; they shall be
evil spirits on earth, and evil spirits shall they be called.
30 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, hunger and thirst, and cause offences.
31 And these spirits shall rise up against the children of men and against the
women, because they have proceeded from them.