From D&D Wiki
What a nephilim is depends on the type of fantasy, fiction, or setting someone is talking about. Below are different examples of what nephilim are or their creation.
Diablo franchise by Blizzard Entertainment: A nephilim are created from the mating of demons and angels.
The Mortal Instrument series by Cassandra Clare are half-angels, created either by drinking from the Mortal Cup or when two nephilim reproduce, their offspring are nephilim.