I was of the opinion, that you only have to make that check, if you took tadpoles, in f Not, I think without slurping up tadpoles,but should be easier, but with tadpoles, I would make it even harder tbh.
It is only if you use the tadpoles. There is no check if the character has the only one tadpole they started with.
I mean, that was s fair imo.
I think making it a check is fair, but it should occur earlier, for example during the fight at the morphic pool. This is the moment where I'd expect the brain to use evrything it has against the party. As they are now, the tadpoles are just a powergaming mechnics, and adding the check late in the evil path feels a bit 'too little, too late' to be a real consequence of giving into temptation.
Tbh, after playing through it once (from an old save), I have the impression these evil endings are in part leftovers from the original EA story. My guess is the illithid transformation was planned if you gave into the dream lover's temptation, but in EA this came with the bonus content of dream interactions. Now you get the dream interactions regardless, and the endings feel disconnected from the story; it would make more sense to me if using the tadpoles had the character roll against becoming illithid, while not using the tadpoles would make it harder to dominate the brain.