It's important that you never say the words "Mind Flayer" at the table for this to work. (And it's better if you never say it in company at all.)
It's also important that the party never encounter a Mind Flayer.
Should a character or a player say the words "Mind Flayer", the words ring out. They linger a bit longer in the air than words should. They stay on end of the tongue and in the mind like arresting poetry.
The Mind Flayer will appear to flay the mind of they who summoned it.
Doesn't matter where the characters are, or what they're doing. The Mind Flayer will come. Out of the blind spot of the room, from the gap in spatial awareness he'll arrive. A tall dark man whose face cannot be seen from any angle. He wears the penumbral robes of a Sorcerer-King an eon past, and only need reach out and touch the scalp with its too-long finger to send its unfortunate victim crumpled to the floor. It will do this for each person witness to its arrival.
The victim finds themselves in a desert, featureless and bleak, with gray sand and an alien sky of never-moving phosphorescent moons, and stars that don't blink.
There is no one there. There never will be. It is a hell.
They do not age or starve. They will never find anything, ever. Just dunes and unchanging stars. For hundreds or thousands of years, they will find nothing. Until they don't.
They'll find the door. A simple stone threshold. The escape. But will they take it? Finding something after an eternity of nothing is a terrifying prospect. Much more so that the door will have always been there, unnoticed. It will have been lingering nearby, waiting to be found, stalking you through this featureless gray waste.
Walk through the door and you'll return, a few moments past falling to the Mind Flayer. You won't be the same person. Your body will be perfectly serviceable. The mind will have aged, spiritually rotted. The soul has been worn out by time.
The Mind Flayer took it. And then it left.
Speed: Floating speed as Man
Mind Flay: Touch Attack, or Save vs. Wands/Staff/DEX - Become incapacitated, and trapped in the Gray Waste for 1000-(INT*20) years. Drains the greater of 2d20 INT. For Dwarves, or other medium-lived races, drains 1d20 INT. For elves, or other long-lived races, drains the lesser of 2d20 INT.
Note: The time spent in the Gray Waste is simulated time. To anyone witnessing this event, the person will be incapacitated for only about 1d8 rounds.
(This creature might be a bit metagame-y for your tastes. If you already have Mind Flayers in your campaign setting, it may be substituted for Intellect Devourer or Orcus or some other big bad.)