1 And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants. 2 And they became pregnant, and they bare great giants, whose height was three thousand ells: 3 Who consumed all the acquisitions of men. And when men could no longer sustain them, 4 the giants turned against them and devoured mankind. 5 And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another's flesh, and drink the blood. 6 Then the earth laid accusation against the lawless ones. Source

And we affirm indeed the existence of certain spiritual essences; nor is their name unfamiliar. The philosophers acknowledge there are demons; Socrates himself waiting on a demon's will. Why not? Since it is said an evil spirit attached itself specially to him even from his childhood — turning his mind no doubt from what was good. The poets are all acquainted with demons too; even the ignorant common people make frequent use of them in cursing. In fact, they call upon Satan, the demon-chief, in their execrations, as though from some instinctive soul-knowledge of him. Plato also admits the existence of angels. The dealers in magic, no less, come forward as witnesses to the existence of both kinds of spirits. We are instructed, moreover, by our sacred books how from certain angels, who fell of their own free-will, there sprang a more wicked demon-brood, condemned of God along with the authors of their race, and that chief we have referred to. It will for the present be enough, however, that some account is given of their work. Their great business is the ruin of mankind. Source