The angels which kept not their first estate, but left their own habitation, He hath reserved in everlasting chains under darkness unto the judgment of the great day (Jude 6).
This controversial verse provides an important insight into earth history. It is parallel to II Peter 2:4. God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment. These chained angels cannot be the same angels as the principalities and powers of darkness which are still in high places (Ephesians 6:12) under the direction of Satan. They are apparently the spirits in prison to whom Christ preached when He descended to Hades in the spirit while His body was resting in the tomb (I Peter 3:18,19). In the next verse, we learn that they had been disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing (v.20). This is also confirmed by the fact that the term spirits, when used in the plural and without modification, always refers elsewhere to either angels or demons.
Thus it becomes evident that these wicked angels must be the same as those sons of God (Genesis 6:14) who left their own habitation in heaven, disobedient to God in going after strange flesh (Jude 7) in the days of Noah, possessing the bodies of ungodly men and women and hoping thereby to corrupt all flesh on earth (Genesis 6:12). The terms sons of God in the Old Testament always refers only to angels, but these angels kept not their first estate (principality), and through the medium of demon possession (not angel/human cohabitation), attempted to defeat Gods purpose for mankind.
However, this crime merely sealed their own doom. God did have to cleanse the polluted earth with the global waters of the flood as a result, but neither man nor angels can ever destroy Gods holy purpose in creation. HMM