Illithid, also known as mind flayers, are tentacled monstrosities that are generally weak of body but strong of mind. Their unmatched psionic attacks can paralyze an opponent, giving the illithid ample time to wrap its tentacles about the victim's head to suck out their brains. Most heroes die from the procedure. Mind flayers are also capable of creating realistic illusions that are rather difficult to disbelieve. They live in the moist caverns and cities of the enormous Underdark. Illithid believe themselves to be the dominant species of the multiverse and use other intelligent creatures as thralls, slaves, and cattle.
Some illithid societies feature inside their cities a huge mass of brain, which serves as the society's heart, memory and most usually their ultimate leader. All illithid within the society are connected to the massive brain via their innate psionic abilities, thus being able to insert knowledge into and from the brain. The mass also serves as an effective alarm system; should one of the illithids be attacked, the others would be warned immediately. Slaves are often placed around the brain to protect it, clean it, and even massage it.