1) "Oh wait i know a guy, he should probably be somewhere around this city ... i mean, yeah he can basicaly teleport around the world, but could we please put the apocalypse on hold for a few minutes so i can go check if he is still aviable?"
Accepted.
2) And this is maybe even more important question ... since we DO know now, what kind of character is the Emperor:
If he would get another Illithid ally ... far more capable and fitting to his plans than us ... why would he need us still? O_o
Like a team of fighters to take him to Elder Brain. He's going to use us as a battering ram. To oversimplify, the Emperor's whole plan is to get to the Elder Brain using a militant team with the leader holding a prism. One Omeluum isn't going to replace an entire SWAT team. Besides, there's nothing stopping him from combining two in one - another Illithid ally and a SWAT team, and Omeluum would be an ally for both us and the Emperor (especially since the Emperor and we're allies up to a point), so why choose and give up anything? And whose side Omeluum would take if we decided to free Orpheus is a separate question. But yes, all of this requires at least bringing Omeluum into the camp to begin with.
