The Book of Enoch and the Watchers
An ancient text that did not make the final cut of most Bibles, in which two hundred angels called the Watchers descend to Mount Hermon, take human wives, and then — this is the part everyone remembers — teach humanity metallurgy, cosmetics, weapons, and astrology, as though the fall of civilization were largely a continuing-education problem.
Why it was classified
Not suppressed so much as edited out. The book was widely read in the centuries around the start of the common era — fragments turned up among the Dead Sea Scrolls at Qumran — and it is quoted directly in the New Testament Epistle of Jude. It nonetheless fell out of the Jewish and most Christian canons. The one place it never left is the Ethiopian Orthodox Tewahedo Church, which kept it in scripture, in Ge'ez, for well over a millennium while the rest of the world assumed it had vanished.
Why it matters
Enoch is where a surprising amount of later Western demonology comes from: named fallen angels, the giant Nephilim offspring, the imprisonment of the rebels, the flood as cleanup operation. Note also what the text is not. The 'Dark Watchers' occasionally attached to it in modern retellings are a separate thing entirely — tall shadowy figures reported on the ridgelines of California's Santa Lucia Mountains, described by John Steinbeck and by the poet Robinson Jeffers as local folklore, with no connection to the Ge'ez manuscript whatsoever. The two got stapled together on the internet because both contain the word 'watchers,' which is roughly the standard of evidence involved.