Monstrous Arcana: The Illithiad

Omens wrote in about Mind flayers:
I am trying to collect information about these fiendish creatures and i was wondering if there is any product that was specifically detailling it.
So do anyone know if such information is hidden inside a specific module or a rulebook ?

I recently read the Illithiad, and found it interesting.  While it indeed explains/defines Mind Flayers themselves, I would say the majority of the book highlights additional creatures related to and similar to Mind Flayers, their weapons, their cities, their writing, and things like that.  It can be bought for as cheap as $20.00, so, even if you don't like it, it's relatively inexpensive.  

The mind flayer takes its prey by means of psychic interference, making the hapless victim believe that its surroundings are innocuous rather than deadly. Someone whose brain is being eaten by a mind flayer might for instance experience the illusion of reading an online discussion about mind flayers, when in fact YOU ARE LYING ON THE FLOOR OF THE DUNGEON WITH SLIMY TENTACLES DIGESTING YOUR CEREBRUM!!!!

Your only hope is to attack the mind flayer itself, which would typically disguise itself as a desktop computer, smartphone, or similar device. SMASH IT NOW, WHILE THERE'S STILL TIME!!! DO IT, QUICK!!!!

I have some experience with them and will relate what little I know.
Illithids are an ancient race; the term illithid is a partial anagram of the name of the demon goddess Lilith, the progenitor of the species. They are able to alter their appearance via a permanent 'polymorph self' spell. Initially, the creature typically uses a form which is attractive to its victim. It is able to read the victim's thoughts, and react accordingly in order to nullify any effective defense. Reproductive activities are the common lure, and although the creature derives no enjoyment from it, it will often simulate pleasure as a part of the luring process. The creature typically recasts the polymorph spell annually, in order to disguise its true nature by simulating aging and weight gain, thereby avoiding detection.
The process of brain extraction as described in texts is incorrect. It is a highly intricate process: first, they conduct a ceremony between the flayer and the victim during which a confining band of metal is placed on the victim's finger by which he is controlled. Over time, other binding processes are created, such as producing offspring (which can either be human or illithid). The flayer commonly instructs its victim to perform certain menial tasks; this is thought to be a method of concealing the bran consumption by occupying the victim's remaining faculties.
The actual consumption of the brain is a slow, painful process that can take years, but as expected eventually leaves the victim a mindless drone. The brain is not extracted intact, but is slowly eaten away over time, and only the parts dealing with higher functions such as reasoning and intellect are devoured (cortex and cerebellum). The stem is left intact in order to keep the victim alive. The process is not visible; microscopic portions of the brain are transferred from victim to flayer via a special conduit which traverses the Ethereal Plane
The period of time between the initial attack and final stage of extraction varies wildly; however, flayers have been known to consume multiple victims in a very short period of time. Occasionally, for no known reason, they will abandon a victim before extraction is complete. The victim often recovers if the extraction has not progressed far, but in most cases there is permanent damage. The damage a victim has suffered can easily be assessed by the general attitude of the victim towards its assailant. As the damage progresses, the victim shows increasing signs of irritation, bitterness and a lack of interest in life.
Unfortunately, victims are not usually immune to further attacks from the same flayer, and competition between flayers for the same victim is common. Often they will congregate in social gatherings and communicate telepathically, however, there exists great rivalry between individuals and it is considered a sport to "steal" the victim of another flayer.
There is no known pattern to the choosing of victims; some flayers will spend an entire lifetime consuming a single victim, while others will consume multiple ones over the course of their lifespan. Their life expectancy is nearly identical to humans. In death, they retain the last form that they utilized.
I would add more, but I seem to have forgotten much of the lore. If I recall any more details I will relate them, but I must log off for now. My wife has asked me to do the dishes, take out the trash, change the O2 sensor on the car, fix the railing on the stairway, and numerous other tasks.

hi
there's also Ravenloft thoughts of darkness module another cheapish module
re4321

copycat wrote in about Mind flayers:A few other things, from 2nd edition, that have mind flayers predominantly involved:

A Darkness Gathering
Dawn of the Overmind
Masters of Eternal Night

I wish I could trust you, but at this point we don't know who among us is the mind flayer. It could be any one of us, & all this talking isn't going to get us out alive. So look, I think we should all agree to submit to a simple test. I'll think of a number, & then the rest of you guess what number it is. Whoever guesses it correctly must be a psychic parasite.

Omens wrote in about Mind flayers:Hello everyone.

I am trying to collect information about these fiendish creatures and i was wondering if there is any product that was specifically detailing it.
So do anyone know if such information is hidden inside a specific module or a rulebook ?

Not a Module or Rulebook but one of Roger Moore's Ecology Series detailed the Mind Flayer Issue 78, Oct 1983 - 3 pages

and in Issue 150, Oct 1989 you'll have the The Dragon’s Bestiary by Stephen Inniss that details the other creatures typically found with them - 4 pages

Two Minutes to Midnight is the first song I play daily.

Deadlord wrote in about Mind flayers:Two Minutes to Midnight is the first song I play daily.

Did you get a chance to go and see Maiden a couple of weeks ago at the Xfinity Center (Mansfield, MA)?  Epic show!