What...Have you heard of this before?

Well, it did happen once a long time ago. Genesis 6:1-2 says, “And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God [angels] saw the daughters of men that they were fair; and they took them wives of all which they chose.”

Genesis 6:4 goes on to say that the offspring were giants, mighty men, men of renown. I believe that it is from stories of these half-demon (since demons are fallen angles)/half-human that we get legends from Greek mythology.

This was Satan’s attempt to corrupt the human race in order to thwart God’s plan to send a sinless redeemer (Jesus Christ) as mentioned in Genesis 3:15. (Note that Genesis 3:15 mentions the woman’s seed, not the man’s seed. This is a reference to the virgin birth of Jesus Christ.)

The half-demon/half-human offspring were all destroyed in the Great Flood (Genesis 7:21-22).

I do not believe it can happen again since the angels that were guilty of this act are “reserved in everlasting chains under darkness unto the judgment of the great day” (Jude 1:6).