I'm not sure the Emperor was originally meant to be in The Adversary role, however there is an illithid lore tablet on the tutorial ship that references it and at that point it is obviously there to clue the player into who the Emperor ending up being in the final game. By the end of the game it could be The Emperor, Tav/Origin, or even Orpheus or Karlach. However in terms of illithid "culture" for lack of a better word The Adversary is more of a boogieman about the one that will end ALL their efforts, not just one colony's like in BG3. There's room for it to be taken as something of a religious myth, even though they genuinely fear such an entity, to help the Elder Brains maintain control of the colonies as they've been doing it for so long they no longer want to give that control up. By not allowing the mindflayers to develop personalities they're basically guaranteeing that the Adversary never appears, which is also why those born with arcane power often end up killed by the colony.
There's also the theory that the illithid empire never actually existed and therefor was never actually overthrown... YET. And if so when the empire does rise, The Adversary will with it. That theory also causes some VERY interesting questions about how illithid colonies currently function with the Elder Brains in complete control, which in this scenario can be read as their own unwitting self-sabotage. Because for their empire as it's known to rise at all in this case mindflayers do need far more developed self-identity which requires far less direct control over them.
i think the meaning of the Adversary, is the original host's will control the illithid after the transformation?
this means the Adversary is an other race that control an illithid body to slay an elder brain... how awful to an elder brain!
however, emperor just owns the host's memory, the will is from the illithid self.
