[Mind Flayers are obviously the most iconic "M" psionic monsters, but I have already done a detailed post on them, http://oldschoolpsionics.blogspot.com/2011/01/psionic-megamonster-mind-flayers-sith.html]
A Marilith is an awesome psionic monster, a demoness in fact. Unfortunately, while you are busy trying to check out her bodacious double D's, she is going to be chopping your head off with one of her six swords...
Basically, Marilith are the perfect physical manifestation of the Feminist Ideal. Let me quote directly from the 1e MM on this one:
"Lower level [males] greatly fear the domineering and cruel [feminists].... [Feminists] of this type are likely to desire the sacrifice of strong warriors to them."
See what I mean? There is more:
--They slither around looking like strippers, and they can Charm Person at will, but all they really want to do is sqeeze the life out of you...
--They are experts at looking great, making appearances, and causing sparks (polymorph self, project image, cause pyrotechnics)...
--No, you CANNOT touch them (AC: -7)!!!!
--They are, however, vulnerable to sweet talk, especially if you use their right name... ("All of these creatures have names which can be used to aid in negotiations.")
--They are chaotic evil! 'Nuff said.
Frankly, with so many attacks, with such a low AC and such a high magic resistance, combined with their psionic and magic powers, if your players don't handle a Marilith encounter skillfully, they could very easily be looking at a TPK. Your best bet is definitely to mind blast them into submission.
attacks: 6 hand weapons and 1 tail (2-8)
Magic resistance: 80%
Psionic Ability: 130
attacks: Psionic Blast, Psychic Crush
defenses: Mind Blank, Thought Shield, Mental Barrier
cause darkness, 5' rad
levitate (1100 pounds)
detect invisible object
gate demon (50%): type 1 (30%), type 2 (25%), type 3 (15%), type 4 (15%), type 6 (10%), lord or prince (5%)