The mind flayer is a classic AD&D monster that has terrorized campaigns around the world for decades. Different origins, variants and alternate versions of the mind flayer have been designed for various RPGs, especial since the illithids have been deemed as product identity by WotC. But I've never seen this one.
Mind flayers are an advanced form of undead that wield strange powers due to the brains they eat.
The mind flayers once existed as any other race until they were infected by an ancient contagion. The contagion increased their psionic abilities but gave them an insatiable craving for the brains of other intelligent beings--including their own race. Mind flayers can no longer procreate normally. They reproduce by infecting a promising victim with some of their own flesh. The contagion takes over and the new mind flayer is brought underground to his or her new people.
This idea has some other possibilities:
- There were mind flayers that were not infected by the contagion that corrupted the rest of their race. They may be hiding anywhere in the campaign world. They may be a hidden force of good, or a bitter enclave just as evil as their brethren. The other races will confuse them for their brain-eating brethren. Plus, other mind flayers have not tasted fresh mind flayer brain in sooooo long...
- Some of the undead in the campaign may be created by the mind flayers as vehicles to get more bodies and brains. The zombies eat their victims, the viruses animates the bodies, and the mind flayers render the zombies to get the tender brains and flesh. Mind flayer zombies probably wouldn't be turned by a priest, and would be very creatively designed.
- The relationship between mind flayers and necromancers can go in some interesting ways. They could be wary allies, traders in various resources, or bitter enemies. Powerful necromancers may know formulas that allow them to control mind flayers, or the contagion that changed them.
- Mind flayers and the undead they create would probably be parallel to the undead created by other means, There may be wars with liches and vampires over their shared food source.