Stuff related to the adventure;
https://www.completecompendium.com/appendix/mindflay/
Psionics fumbles from the 2e handbook, these would effect mind flayers of course . They should all be psionicists and not mages (having magic makes them stronger as magic has no fumbles) as
the only reason they can also have magic is because not everyone had the psionics handbook. They also have 90% magic resistance. It also maintains and enhances their weirdness.
Fumble results follow for the typical Mind Flayer powers. The book states "Both results are optional. DMs can use these results - or devise something similar - to add spice and a small element of risk to psionics" as there are positive and negative effects but its more fun using them both than not.
control body; The psionicist suffers partial paralysis (an arm or leg) for 1d10 turns.
levitation; The psionicist doubles his weight for one round. If he falls, he
suffers an extra d6 points of damage.
body equilibrium; The psionicist's weight instantly rises by a factor of 10; he must use
body equilibrium to fix the problem.
probability travel; The character attracts the attention of an astral creature, which may
or may not be hostile, at the DM’s discretion.
teleport; no effect
astral projection; The psionicist attracts the attention of a powerful creature in the
astral plane.
domination; The victim knows that someone attempted to dominate him
mindlink; The psionicist must save vs. petrification or be stunned for 1d4
rounds.
awe; All contacted characters view the psionicist as pathetic and
ridiculous.
contact; Further contact with this mind is impossible at this experience level.
ESP; The psionicist develops a splitting headache and suffers a -1 penalty
on all telepathic power scores for one hour.
ego whip; No additional effect.
id insinuation; The psionicist's id is unleashed; the resulting rage imposes a -1
penalty on all his attack rolls and power checks for one turn.
post-hypnotic suggestion; The intended victim is aware of the attempt.
Since this is ravenloft, then it would be great if a DM added more "mutates into ogre-like mind flayer tentacle horror" or something like "turns into a slime puddle with pseudo-pods barely resembling tentacles for movement" and other fun stuff. These would effect nobles and the elder brain of course.