I think Corax would be both very darkly amused and very confused as to who the fuck this mortal is that they can threaten gods and be certain they can follow through on those threats
Hmm... there's a thought. If he is thinking 'mortal' the natural assumption would be psyker, a very scary psyker who can go on a jaunt into the warp and not go mad. Which might bring to mind the only other god-like psyker he knows. Who says there is only one of whatever the Emperor is?