[SixOfSpades]

I actually found that it doesn't matter if you kill the Mind Flayer in the room where you get that fluid: I was exploring the place, whacking enemies, and was stopped my doors that said "Only a being with godlike strength or a powerful mind can open this door." Then I looked around for a Mind Flayer to open the thing for me--and I'd already killed them all. But not really--I found that an extra one spawns in the room with the tanks (north of the slaves) so I just Charmed him to do my dirty work. (The Dire Charm wears off when they open a door, but if you've got another collar you can just do it again.)