What did the fallen angels teach humanity?

The fallen angels taught humanity a range of forbidden arts that corrupted the world, including how to forge weapons and work metals, the use of cosmetics and ornaments, sorcery, and the secrets of the stars and heavens. Azazel specifically instructed men in making swords, knives, shields, and breastplates, along with knowledge of the earth's metals, bracelets, ornaments, antimony for beautifying the eyelids, costly stones, and colouring tinctures (The Book of Enoch 8:1-4).

Other Watchers shared further secrets: Semjaza taught enchantments and root-cuttings, Armaros the resolving of enchantments, Baraqijal astrology, Kokabel the constellations, Ezeqeel the knowledge of the clouds, Araqiel the signs of the earth, Shamsiel the signs of the sun, and Sariel the course of the moon (The Book of Enoch 8:1-4). These teachings also encompassed metallurgy for weapons and ornaments, cosmetics and dyes, sorcery, and the secrets of the stars and clouds, turning heavenly knowledge into an engine of violence, godlessness, and moral decay among humanity (Themes: The Watchers and Their Fall; Commentary - The Book of Enoch 8).

In this way the Watchers revealed what was hidden and led the children of men into sin, accelerating the corruption that made the Flood necessary (Facts - The Book of Enoch 64; Theme Facts - watchers).