Created angels in the Old Testament were referred to as sons of God. Understanding that Jesus Christ was the only begotten Son, meaning He was the offspring of God, not created, making Him God. As the Father is, so is the Son, God.
Some of these created angels left their habitation, their angelic bodies, and started living in a fleshly body here on earth. The mechanics of how this happens is not known to us. They did this for the purpose of trying to create their own eternal offspring, man is the only created being that is able to reproduce an eternal offspring. The devil now has his children. We do know this was very wrong and those angels are now held in chains until Christ comes to judge them. We are told this in the book of Jude.
Some of these offspring were born very large, hence giants, and were called Nephilim. And they produced offspring who produced offspring and so on. The devil’s children.
The big question is, are there still offspring from this bloodline that still exist today? And who would they be?