https://forgottenrealms.fandom.com/wiki/Skull_of_the_Void The Mindflayers from the Skull of the Void are known for their ability to levitate, an ability granted by eating a lot of Beholder brains. The Mindlayers in BG3 are seen levitating, perhaps showing a link to the Skull of the Void, Mindflayers as well. It makes sense that there would be a connection between the Mindflayers of the, Shull of the Void and Glyth. Both are in realmspace.
Oh damn, good pickup! So if beholder's show up in BG3, at least the Illithid would be able to advise on how to deal with them. That would honestly be a great 'here's how powerful they are' scene. A group of Illithid with a mutilated beholder floating along in a cage behind them. Like "yeah we messed up this eye tyrant, think we're scared of you?".
And then the beholder gets free and destroys everyone.
Good times.
