Nephilim are children of angels and mortal women. This was expressly forbidden by God, and so they, along with their mothers and fathers, were routinely killed. That is, if the nephilim didn't kill themselves first; they're not the most stable of beings.
Another name for them is "Abomination".
Stark is a nephilim. Currently, he is the only one.
The powers of a nephilim differ. Some are more human, while others are almost angels.
It's unknown as to whether nephilim have souls.