You mean an actual showdown of a massive alien interstellar empire versus a demon?
Diablo gets crushed.
I mean, I get that he's the lord of evil and stuff, but really, what's he going to do against a kilometer long frigate with guns that deals almost a hundred megatons of destruction every second and has both shields and hull designed to withstand that kind of punishement?
Let alone an entire fleet of these...
... and then a 20km Titan packing several times over that amount of firepower, a dozen 5km long capital ships also packing massive firepower, massive star bases, etc...
Not to mention in the end Diablo gets killed by one lone hero, so yeah.
He won't even scratch the paint job on a scout frigate.