The Nephilim were a race of often man eating giants described in the Christian Bible and were sometimes depicted as the hybrid offspring of "the sons of God" and the "daughters of men. The "sons of God" were actually fallen angels particularly known as the Grigori.
The Nephilim were fearsome creatures and took to terrorizing the other creatures of the Earth, foremost amongst them humanity, this behavior was unacceptable to God who, according to some sects, decided to send forth the Great Flood to rid the world of the abominations before they could do more harm to humanity.
Some of the Nephilim, or their descendents, did manage to survive the Flood. Or perhaps it is possible that more Nephilim were born from unions of human females and angel. It is later revealed that some of the Nephilim did indeed survive, albeit in great smaller numbers, after the Flood but were no longer considered a threat to God's creation. In some texts the ancient Hebrews encountered some of these surviving Nephilim on their way to the Promised Land.
Overtime, the surviving Nephilim were slaughtered by the humans themselves, or more specifically human tribes that sought out to drive these giants from their lands. While they were regarded as Nephilim, they were in actuality, the decendants of the Nephilim of old age that survived the Great Flood.
While many scholars believe that the Nephilim were a myth, excavations say otherwise due to the fact that many have been finding humanoid skeletons buried deep in the Earth, and some were even 15 to 20 feet tall in height.