One of the german CMs confirmed that it actually is Rathma …
He could be mistaken, but unless there is a official correction of this statement, I would say that the guy in the cinematic is Rathma, the son of Lilith and Inarius.
Edit: Oopsie, this Bluepost was indeed nothing official. He edited his posting to make it clear that the real identity of this guy is not revealed yet:
I might have been a little hasty posting here! In fact, I was only resorting to my own nerd-knowledge and wasn’t aware at the moment of eagerness that my blue writing might suggest more. For now, the identity of this ominous gatekeeper remains a mystery~