The opposition arrives in a puff of malice. Guardians of the revolution, the Watchers keep an eye on Hell's denizens. Long ago these Infernal agents dwelled on earth and mated with women, creating the Nephilim giants, to whom Goliath was supposedly related. Most of them were drowned in The Flood. 

Note I am not trying to create something that rigidly adheres to religious accounts. Rather, I intend to use Hell as a setting and framework with which I can tell a story about something else entirely. So there is no point in objecting to deviations from religious accounts.