Scripture states the fact of an angelic fall without narrating the event. Jesus speaks of "everlasting fire, prepared for the devil and his angels" (Matthew 25:41) — Satan has angels, and their doom was prepared for them, not for men. Peter writes that "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 Peter 2:4). Jude describes "the angels which kept not their first estate, but left their own habitation," now "reserved in everlasting chains under darkness unto the judgment of the great day" (Jude 1:6).
That is the core testimony: some angels sinned; their sin involved abandoning the position God assigned them; they are certain of judgment. Scripture does not tell us when the fall occurred or exactly how many fell. Some point to Revelation 12:4, where the dragon's tail "drew the third part of the stars of heaven" — and stars can symbolize angels in Revelation (Revelation 1:20) — but that verse sits inside a vision of conflict, and careful readers differ on whether it describes the original angelic fall at all. Where the text is reserved, honest teaching is reserved too. What no reader can miss is the moral: if God "spared not the angels that sinned," sin is not safe for anyone (2 Peter 2:4-9). There is also no redemption offered to fallen angels — Christ took hold of "the seed of Abraham," not the nature of angels (Hebrews 2:16). Grace toward sinners of Adam's race is not owed; it is astonishing.