I decided to replay the OC with a wizard character on very hard difficulty setting...
I struggled in the beginning a bit, but then it got better when I reached the fifth level and got better spells.
And so I got an intellect devourer's brain. Or so I thought.
Frankly, I recall retrieving the brain from the remains and teleporting to Aribeth and company to progress. But I couldn't find any related conversation options... and then I checked my journal, which had no info on me defeating the devourer and taking the brain.
And then I finally checked my inventory to notice that I don't, in fact, have this brain with me. I'm not sure how this could have happened (I really DO remember taking it with me, and I checked to see if there's anything lying on the ground in the devourer's chamber, nothing was there) but I don't feel like starting all over and I only kept two save files, both of which have been overwritten after killing the devourer.
Any advice on what to do?
Edit: Ok, I dealt with the problem by loading the save file as a DM, placing Devourer's Brain on the ground, then logging again as my character in single-player and retrieving it.
Guess it will remain a mystery what exactly happened to the original one.