 The Nephilim (somewhat different spelling from that used in the  series) are mentioned only briefly in the Biblical book of Genesis,  and in more detail in several Apocryphal books including The Book of  Enoch and The Secrets of Enoch. In those texts, the Watchers are a  troop of angels tasked with watching over and protecting Mankind.  They become infatuated with mortal women, descend to Earth and have  their way with them, spawning a race of gigantic monsters, the  Nephilim. For the sin of lust, the Watchers are banished from Heaven  and are condemned to roam the Earth for eternity. Out of revenge,  they teach Mankind the arts of magic and warfare. Their leader is  sometimes called "Azazel" or "Azaziel".