It is all very easy, listen up :

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Somewhere there's a room where you can find vails of dark smelly stuff. Gather some of it and go to the room where the slaves are captured. Free them and pour the smelly stuff into the machine. It will make a few collars you can use to enslave the Brain bugs. Just put it in yer quick items and use it on them, but be sure to have "improved haste" or "Mind shield" casted, so it wont charm you. When you make 'em move near a door, it will open, and the Flayer will turn on you.

Easy as that.

I can't seem to get the door open in the NW portion of the Ithilid Underdark cavern. It keeps telling me "a force prevents a mere "mortal" from openening it" or something to that effect. Dire Charmed ithilids don't seem to open it and the door to the Underdark refuses to open as well. Help please.

P.S. I'll probably feel really stupid after hearing the answer, but still......

I believe it can only be opened by a 'brilliant mind or Godly strength'. Yes charmed Mind Flayers do open it (but I had problems with Ultralithids or whatever they're called)

Otherwise, do you have anyone in your party that can get 24 strength hmmmm? You'd be pretty beastly with that kind of power, SLAY anything in sight I'd think.......

I haven't gotten this far, but if I remember correctly someone said that somewhere down there you are able to make some mind control collars that will allow you to control an Illithid who can open it for you.