I made a post (https://www.reddit.com/r/BaldursGate3/comments/15k7xg1/comment/jv4ejtz/?context=3) a couple days ago about how my game kept ending in Act 3 and I think I found the reason and a solution.

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So I redid some of the fights and I think I figured out what is causing the game over to the Elder Brain! From my understanding you have a finite amount of uses of the illithid powers you can obtain. For example I'm using the Psionic Backlash quite a bit during fights as a way to ping spellcasters and possibly execute them. The amount of uses varies on the fight, but we'll say it averages about 5-6 per fight. Which brings me to my point. I think there is a finite amount of uses of the Illithid powers, for example say you're allowed 100 uses of the powers. Once you reach that 101st use it game overs you and the Elder Brain transforms you into a mind flayer.

That's my current prevailing theory on the powers, but I have no idea what the actual cap is or if the amount of tadpoles you have effects said cap. I also know from my current point in the game I only have 6 uses left of the powers before I'm transformed, so the rest of the game should interesting in adapting to not using the powers I've been abusing since Act 2ish.

TL;DR: Don't overuse the Illithid powers or the Elder Brain finds you and transforms you into a Mind Flayer for a quick Game Over in Act 3.