I've had a fascination with nephilim for about 10 years or so. I discovered them at a strange time in my life. I was dealing with some issues (like the fact that I was an adult that knew nothing about my biological father), growing independent, coming up with my own views on religion, and I'd just met a girl who'd change my outlook on life forever. These thing just led me to grasp on to the idea of the nephilim (or at least my idea). It's stuck with me since (as you can tell by the name of this blog).
Nephilim are the children of Grigori (watcher angels) and the wives of man. To learn more about the angels that sired them, you should check out fellow A-Z blogger, Tim Brannan over at The Other Side. His K post is quite relevant to my post today.
Biblical nephilim were giants among men. Goliath is the first that comes to mind. Modern versions of the nephilim tend to drop the giant aspect and the Grigori parentage. These nephilim are the children of any angel that comes to Earth (fallen or not). I've also seen nephilim used to describe angel blooded characters (like D&D's aasimar) in books and film. I just finished the reboot of the Devil Mary Cry video game series and its version of the nephilim are half-angel/ half-demon.
In the World of Darkness
|DmC: Devil May Cry's nephilim Dante|
Since Grigori are fallen angels (which makes them demons) you could use the possessed rules in Inferno to stat up nephilim (though these nephilim will be extremely dark). If you don't want to go that route think about giving a character some of the following merits out of the core rulebook:
Some apocrypha also claims that the nephilim are the children of Seth or Caine, which could connect them to vampires.