The children of men began to multiply upon the earth and the daughters of men were very beautiful.  And a group of Holy angels called the Watchers were tasked with watching over the humans and they also noticed the beauty of the daughters of men.

So during the life of Jared who was sixth from Adam, 200 of the Holy Watchers from above came down to live among the humans on the earth.

Then these angels began to take wives for themselves of the daughters of men, the Watchers took any they wished and they taught their human wives to beautify themselves, sorcery, charms, enchantments and spells.

These Watchers also taught humans the signs of the stars and their paths, how to write, the making of mirrors, bracelets and jewelry, swords, knives and armor.  Then the Watchers that had come from above slept with their human wives and their wives gave them children.  

The children of the Watchers grew tremendously and became giants upon the earth.  These giants, the children of the Watchers, became known as the nephilim and the children of men were exceedingly afraid of them because they were very strong and very fierce and very wicked just as their fathers are.