He's not well-addressed in the Warborn storyline. But that might have something to do with the Warborn wavering in terms of consciousness. From my understanding, the Abyssal Skills are supposed to come with Orchidna lore and they might go deep enough as to address who Damian is. The only understanding I have of him is that he's Orchidna's son; somehow her biological son given the way she treats him. That's about all we have though. :/ Unless the Abyssal Skills tell us more.
Also, the release of the Warborn and Dwarves is just part one of the Revelations release. There's more coming in...March, I think? So I'd expect that more will be revealed.