The Nephilim (the singular form is Naphil), also called "Watchers," are a biblical antediluvian race said to be born from the "daughters of men" and the "Sons of God." A reference to them is made in Genesis 6:1-7. Nephilim is usually translated into "fallen ones."
There is much speculation as to what the nephilim really are, as nobody is really certain what the phrase "Sons of God" alludes to. While some theologians argue that the term "Sons of God" merely refer to godly or righteous men, others believe that the best interpretation is also the most obvious: a naphil is a result of interactions between the earthly and the divine. Of course, what came of this union is also under debate. A naphil is described in turns to be a giant, a monster, or a half-angel hybrid.
The prevailing myth, however, would lead us to believe that the definition begins with the story of fallen angels. The originators of this myth cite both biblical text and the apocryphal books of the Dead Sea Scrolls, specifically the Book of Giants, to bring credence to their theory.
This story tells us that a high-ranking angel called Shemhazai led a group of followers down to Earth to instruct mankind into the paths of righteousness. After a few centuries, perhaps due to staying on earth for so long, the angels found themselves developing very human passions and started to look at the women of Earth with men's eyes. They mated with these mortal women and produced children, which are the Nephilim.
Here the fate of the Nephilim seems to split. Some texts seem to state that the Nephilim became heroes or great men and left it at that. Others contest that although the nephilim were strong and powerful, they were also drawn to sin. They were said to have staged a rebellion against heaven, incurring God's wrath. Finally, when their wickedness threatened to spread across the earth, God ordered the angel Gabriel to incite a war among them, which eventually annihilated their whole race.