It is my personal belief that any time that God had to destroy anything like during the flood, Sodom and Gomorrah, or specific instructions to the chosen people it was because Nephilim were somehow involved. They even intermingled with animals. The book " The Nephilim and the Pyramid of the Apocalypse" by Patrick Heron talks about some of the things they were involved with. Not much information has been available about them because they are spirit beings therefore invisible but like to operate in darkness and secretly.I believe that this passage in Ephesians 5:11-12 which says "Have nothing to do with the fruitless deeds of darkness, but rather expose them. (12) For it is shameful even to mention what the disobedient do in secret," refers to Nephilim. Some Nephilim children, giants, died during the flood but fallen angel Nephilim did not die. Also the term Nephilim sometimes refers to them or their children.Gen: 6:4 says they were on the earth in those days- and also afterward. They the fallen angels are still influencing people in the world. That is why Paul said in Ephesians at 6:12 " For our struggle is not against flesh and blood, but against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. That is why he goes on to say we need the full armor of God.