I'm stuck.

There's nothing else to it: gone to all the places i can go, except for the the door which says that "mere *mortals* are unable to open the door". I've freed the slaves, got the brine potions against psionic attacks. Spoliers for this !@#$%^&* bit of the game welcome.

Spoiler (maybe)
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In the slave quarters there should be a machine that produces head bands that can be put on mindflayers, to make them subservient. Put one in your quick use slots, find a mindflayer, and "use" it. The mindflayer can then be commanded to open the door for you.

Could you be more specific as to where this door is eg from the cells, where is it? North? south? east? west? If it's the exit door I think you need to kill the elder brain for it to open but try and give me a bit more detail. What was in the room that this door is in?

You could also turn into the slayer! Say no more.

I got cought using all those mind controlling devices before I was home safe and now I have to play the whole quest over again because I'm protecting my rep. with that slayer neg impact on my rep. Not nice!