there was a thread about the mindflayer world in FR up a while ago, basically theres a variety of places mindflayers can come from and several of them could in theory possess a nautiloid
Well, forgotten realms wiki's claims that mind flayers. or at least their ancestor, originate from Far Realm. And while we touched upon the issue of information availability on wiki's, you should definitely leave your mark in history by registering on divinity fandom wiki and help out with filling it with information from Divinity game series. There also a rumor going on that if you will create 3000 pages with content on this wiki, you will get real life psionic powers. But no one managed it so far to prove it or disprove it.
Back to Baldur Gate 3, would be nice if players will get an opportunity to become llithids themself.