Fallen angels are those angels who rebelled against God along with Lucifer, an archangel who later became known as the devil.



"How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, You who have weakened the nations! 13. "But you said in your heart, 'I will ascend to heaven; I will raise my throne above the stars of God, And I will sit on the mount of assembly In the recesses of the north. 14. I will ascend above the heights of the clouds; I will make myself like the Most High,'" (Isaiah 14:12-14).



The angels who fell from Heaven became demons, abiding in Hell with Lucifer- the most renown fallen angel, and the Devil. 



"And another sign appeared in heaven: and behold, a great red dragon having seven heads and ten horns, and on his heads were seven diadems 4. And his tail swept away a third of the stars of heaven, and threw them to the earth . . . " (Rev. 12:3-4).



In the future, there will be judgement of all the humans upon earth, and all of the fallen angels.



"Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels," (Matt. 25:41).

"For if 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," (2 Pet. 2:4).
"And 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 1:6).
"And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him," (Rev. 12:9).



• ​Satan, though capable of disguising himself as an "angel of light", is the leader among the fallen angels. 
• Other references to Satan are ancient serpents and horned demonic figures. Other names include- Lucifer, the Devil, and of course Satan. 

In the New Testament, there is a great red dragon whose tail swept a third part of the stars of heaven and cast them to the earth. In verses 7-9, after defeat in a War in Heaven in which the dragon and his angels fought against Michael and his angels, "the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world - he was thrown down to the earth, and his angels were thrown down with him."[35] Thus, amongst Christians, fallen angels have been associated with the term “cast out”.