Larian Studios Forums Baldur's Gate III Baldur's Gate III - Story & Character Discussion Why BG3's ending reminds me of Mass Effect 3's (ending spoilers, naturally)

And Omeluum is a complete 180 from everything all the NPCs say about mindflayers, even Withers. It's very clear Omeluum is its own entity, and while the Society of Brilliance does seem to be a lawful evil organization (stealing eggs and what not from other species), it doesn't change the fact that Omeluum is everything that we're taught mindflayers are not by pretty much every other source in the game. So much story potential there that isn't given its due later.

As for changing into a mindflayer, what you said is right. At the 'ending' of the game, Larian wanted the story to go a certain way, even though through the entire game, they bent over backwards to let the player define the journey. It's strange and it pops up in a LOT of places. The writers think that it's cool we have this ambiguous or downright melancholy endings, and don't care if players want other 'options.'