Returning to Revelation 12:9,
Rev 12:9 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.
The expression “that old serpent” is of course a reference to the serpent in Genesis 3:1, 4, 13, 15 who tempted Eve in the Garden of Eden.
This is just a small sampling of the many places in the Bible which relate to understanding the fall of Satan and his being cast out of heaven with one third of the angels.
[My answers to some Muslim questions Part 2C]
Some of those cast out angels were involved in the incident mentioned in Genesis 6:1-4,
Gen 6:1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
Gen 6:2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
Gen 6:3 And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
Gen 6:4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
These angels and their fate are mentioned by Jesus in Matthew 25:41,
Matthew 25:41
41 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:
King James Version
Also mentioned by Paul in Ephesians 6:12,
Ephesians 6:12
12 For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.
King James Version
Also mentioned by Paul in Colossians 2:15,
Colossians 2:15
15 And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it.
King James Version
And most remarkably and specifically mentioned by Peter in 2 Peter 2:4 where “hell” is in the original Greek text Tartarus,
2 Peter 2:4
4 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;
King James Version
These events are further mentioned by Jude in Jude 1:6, 7,
Jude 1:6
6 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.
King James Version
Jude 1:7
7 Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire.
King James Version
There is much more in the Bible that relates to this subject, but what I have shared above is enough to give you a good start or beginning to learn what the Bible teaches. The subject is too complex to enable me or any other Bible scholar to give you a one-sentence so-called simple explanation. If your questions are sincere you will welcome the completeness of my answer.
[I had to divide the above material into three separate posts since the site would not accept a post with over 100 line breaks. The first part of the above answer I gave no longer appears on the site where I posted it.]