There are 2 classes of fallen angels.
The first class are the ones now in Tartarus who committed a special sin.
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 wereof old, men of renown.
Gen 6:11 The earth also was corrupt before God, and the earth was filled with violence.
Gen 6:12 And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth.
Gen 6:13 And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
Their goal was to corrupt the original makeup of man, creating half-breeds called Giants (v 4)
There were 2 eruptions of these fallen angels among men, before and after the flood! (v 4)
They broke God’s law of reproduction of “After his kind”
Note: Giants have been judged and will not be judged again at the end of the Millennium.
Isa 26:14 They are dead, they shall not live; they are deceased, they shall not rise: therefore hast thou visited and destroyed them, and made all their memory to perish.
Back to the angels that sinned
1Pe 3:19 By which also he went and preached unto the spirits in prison;
1Pe 3:20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
2Pe 2:4 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;
Jud 1:6 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.
Jud 1:7 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.
The second class are the ones still out there giving Lucifer a hand in trying to corrupt God’s plan.