The Nephilim were a product of the "sons of God" (i.e., "angels") that had sexual relations with human females. The Nephilim were the offspring that came from this evil. What the Nephilim were not were the sons of Seth. If the Nephilim were the sons of Seth then Genesis would have recorded "the sons of Seth" just as easy, but that is not what is written. The Nephilim no longer exist on earth today and pose no threat to our existence.