I agree with the idea that there should be consequences for choices, going back to EA, having consequences for consuming tadpoles was something I looked forward to seeing in the game. But, I also have to agree that going about it this way feels very bad. It feels like they shoehorned this into the evil ending so that they can now say there are consequences, rather than ever working it into the game in a way that at least felt interesting.
Larian loves to gush about how they restarted scenes because someone pointed out that their character wouldn't behave like that and how the game allows such freedom of choice (which, generally is quite good) - until it comes to evil decisions. I think what particularly makes this roll feel so bad is that it doesn't just force the player to become illithid if they don't pass, but that by not passing you completely lose out on choosing how you want your game to end.
If the good option was not to "Command the Absolute to destroy the tadpoles and then itself" then the roll here might feel slightly less bad. Given the feats you can witness of the various origins after they've dominated the Absolute, it feels especially ridiculous that the roll takes place after the player has been empowered.
If you're already a mindflayer, you have to make the 25 DC con check as well in order to enact your own will, otherwise you will succumb and enact the Grand Design. If the game didn't force you to need a mindflayer to complete, maybe this wouldn't feel quite so bad either, but as it stands it ultimately just feels like you're being punished for playing the game but not quite following what Larian had in mind. While I agree that if Larian wanted this to occur, moving it to a spot where it could at least affect good endings would make it feel more thematic and less as a punishment for players choosing the evil ending, I would assume it was going to create too much work to do so. Additionally, it would then come into conflict with Larian's "will you become a monster to save the city?" question the game apparently was always meant to ask. Obviously, they could just make it not work on a player that is already a mindflayer, but then that would feel odd for other reasons.
So, I just don't see this particular roll fitting in with what is already in the game. Perhaps some rewording of the dialogue could make it work, but I'm not sure I ever see it not feeling like yet another thing Larian tacked on without regard to how it interacted with prior gameplay. Otherwise, it does feel like a case where not doing anything would have felt better than doing something poorly.
