Not sure what you mean by Valkorion came out of nowhere. I mean, he did, but hints were dropped left and right at the end of SOR and especially Ziost that Vitiate had something big planned. Hell, it's actually kind of a good thing that Valkorion brain-rekt everyone and seemingly came out of nowhere, given that was kind of the point, as it puts you in the position of the characters, (i.e. being completely brain-rekt and fooled by Vitiate.) And Valkorion sort of explains why he acts like he does, (his new empire gave him liberties he didn't have in the sith empire, he was always hinted to be "beyond" sith ideologies, etc) so that's not too much of a viable complaint either. And him being Sidious tier was sort of always a thing.