It is written:
“And all flesh died that moved on the earth: birds and cattle and beasts and every creeping thing that creeps on the earth, and every man. 22 All in whose nostrils was the breath of the spirit of life, all that was on the dry land, died. 23 So He destroyed all living things which were on the face of the ground: both man and cattle, creeping thing and bird of the air. They were destroyed from the earth. Only Noah and those who were with him in the ark remained alive.” (Genesis 7:21-23)
The Nephilim giants which had enslaved humanity were destroyed in the Flood. According to the book of Enoch, the spirits of these nephilim remained in the world as demons to torment humanity.
1 Enoch 15:8-12-My judgment for the giants is that since they are born from flesh they will be called evil spirits and will remain on the earth. 9 Because they were created from above, from the holy Watchers, at death their spirits will come forth from their bodies and dwell on the earth. They will be called evil spirits. 10 The heavenly spirits will dwell in heaven, but the terrestrial spirits who were born on earth will dwell on earth. 11The evil spirits of the giants will be like clouds. They will afflict, corrupt, tempt, battle, work destruction on the earth, and do evil ; they will not eat nor drink, but be invisible . 12 They will rise up against the children of men and against the women, because they have proceeded from them.
While the book of Enoch is not inspired Scripture, it is very clearly a “recommended reading book” by Scripture. Michael Heiser has well said:
“First and foremost is the matter of canonicity. A handful of important early Christian writers such as Tertullian, Irenaeus, Origen, and Clement either advocated 1 Enoch as worthy of canonical status or considered it authoritative on certain matters of truth and doctrine. The book was assigned full canonical recognition only in the Ethiopian Church….I don’t consider the book of 1 Enoch to be inspired and canonical, but that is no excuse for neglecting it in the study of Scripture. Frankly, this entire book is testimony to the folly of this inattention. The assumption that uninspired ancient books aren’t valuable for understanding Scripture is deeply flawed. Biblical writers in both testaments show detailed knowledge of ancient writings now known to the modern world. That this material wasn’t inspired didn’t bother biblical writers. It is well known among scholars, for example, that Old Testament covenants follow the structure of different types of ancient Near Eastern treaties,[ 7] that prophets and psalmists quote from the Baal Cycle (e.g., KTU 1.5. I; Psalm 74: 13), and that Solomon borrowed material from the Wisdom of Amenemope for Proverbs 22: 17–23: 11. In the New Testament, Paul’s quotations of Greek poets are well known (Acts 17: 28, Epimenides and Aratus; 1 Corinthians 15: 33, Euripedes or Menander; Titus 1: 2, Epimenides) as is the use of the apocryphal (“ deutero-canonical” to Roman Catholics) Wisdom of Solomon in Hebrews 1: 2 (Wisdom of Solomon 7: 26). These are far from the only instances.” (Dr. Michael S. Heiser, Reversing Hermon: Enoch, The Watchers & The Forgotten Mission Of Jesus Christ, 127-138 (Kindle Edition))
It was certainly the teaching of many ancient Jewish sources that the “demons” of the biblical world included the spirits of the Nephilim. For example:
Jubilees 10:1-6 (emphasis added, M.T.) -1. And in the third week of this jubilee the unclean demons began to lead astray the children of the sons of Noah; and to make to err and destroy them. 2. And the sons of Noah came to Noah their father, and they told him concerning the demons which were, leading astray and blinding and slaying his sons’ sons. 3. And he prayed before the Lord his God, and said: God of the spirits of all flesh, who hast shown mercy unto me, And hast saved me and my sons from the waters of the flood, And hast not caused me to perish as Thou didst the sons of perdition; For Thy grace hath been great towards me, And great hath been Thy mercy to my soul; Let Thy grace be lift up upon my sons, And let not wicked spirits rule over them Lest they should destroy them from the earth. 4. But do Thou bless me and my sons, that we may increase and multiply and replenish the earth. 5. And Thou knowest how THY WATCHERS, THE FATHERS OF THESE SPIRITS, acted in my day: and as for these spirits which are living, imprison them and hold them fast in the place of condemnation, and let them not bring destruction on the sons of thy servant, my God; for these are malignant, and created in order to destroy. 6. And let them not rule over the spirits of the living; for Thou alone canst exercise dominion over them. And let them not have power over the sons of the righteous from henceforth and for evermore.”
With this in mind, we see that the forces in the spiritual war against us include: Satan; 1/3 of the angels who have rebelled against God; 200 fallen angels who committed sexual sin with humanity; and the spirits of the Nephilim.
However, the spiritual war becomes even more bizarre when we realize that the Nephilim and their descendants are said to play a major part in the world before the Second Coming of Jesus.