Nephilim. It's right there in the Bible:
Genesis 6:1-4
1 And it came to pass, when men began to multiply on the face of
the earth, and daughters were born unto them,
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.
3 And the Lord said, My spirit shall not always strive with man,
for that he also is flesh: yet
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