The Nephilim were the offspring of demons and human women. They are the result of demons serving under Abaddon and Asmodeus, raping imprisoned human women during the Time of Babel. They possessed the gifts of man and the Lores of demons and were terrible to behold - if not for their hideous countenance, then for their potential. Some may have been born with benevolent, gentle spirits and took it upon themselves to guide and enlighten, but many were born knowing only hate and tyranny. In a few short years, they dominated the cities of their birth. They hunted down and killed the Ten and were then in turn hunted down and killed by Lucifer and his demon followers.
Azrael was the only one not involved with the creation of the Nephilim to discover their origin. He traded his silence for the right to their souls when they finally passed on. These dead Nephilim worked as ferrymen in Haven. What has happened to these dead Nephilim is unknown.
It is unknown if any Nephilim survived to pass on their lineage to the Final Nights, but many demons wonder if any of the supernaturals that populate the modern world are descendants of the ancient Nephilim.