Some believe the Nephilim in Genesis 6 were the offspring of humans and fallen angels. If human, these Nephilim would have all died during the flood of Noah's time, yet the Bible mentions Nephilim living after the flood (Genesis 6:4; Numbers 13:33). Some therefore speculate that the Nephilim who died became disembodied spirits that we now know as demons.
Are demons actually the disembodied spirits of the nephilim?
The only way this could even be possible is if the Nephilim were indeed fallen angels or demons. This is inconclusive, so at the most the answer is only maybe.
In addition, the explanation of the Nephilim as demons is partly related to one's understanding of 1 Peter 3:19 (also in Jude 1:6). First Peter 3:18-20 states, "For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit, in which he went and proclaimed to the spirits in prison, because they formerly did not obey, when God's patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water." What does this passage mean?
Some have sought to interpret this passage as referring to Jesus preaching to fallen angels during the time between His death and resurrection. They presume that all the fallen angels are imprisoned, meaning that what we know as "demons" must be something else. But this assumption makes no sense. Clearly Satan, the leader of the rebellion, is not currently imprisoned (2 Corinthians 4:4; 1 Peter 5:8). Why would all the other angels be imprisoned? It makes more sense that the spirits in prison are the fallen angels who participated in the specific rebellion described in Genesis 6. Or, alternately, it could be referencing Jesus speaking through Noah during Noah's life on earth. Understood this way, God (Christ) spoke through Noah to those in his time while the ark was being prepared.
If this understanding is correct, there is no need to speculate regarding the much more difficult theory that demons are the disembodied spirits of the Nephilim. Instead, demons are fallen angels, those who rebelled against God with Satan sometime prior to the Garden of Eden (Ezekiel 28; Isaiah 14). They do not possess a human body, but instead are spirit beings who influence toward evil.
Ephesians 6:12 speaks of the spiritual battle Christians face with such demons, saying, "For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places." As with Satan, however, these demonic powers are limited in their ability. They can only operate as God allows. In addition, the power within the believer in Christ is greater than Satan or his demons (1 John 4:4).
In the end, Jesus will crush Satan underneath His feet (Romans 16:20), along with defeating Satan's demonic army. They will be judged with eternal separation from God, tormented forever in the lake of fire (Revelation 20:10).
What are demons according to the Bible?
Are demons and fallen angels the same thing?
Nephilim - Who or what were they?
What spirits in prison did Jesus preach to between His death and resurrection? Are the spirits in prison related to the Nephilim?
What are the principalities and powers the Bible talks about?
Truth about Angels