Illithid

The origins of the illithids are often shrouded in mystery, with conflicting stories existing.
The book The Illithiad suggests they may be from the Far Realm, an incomprehensible plane completely alien to the known multiverse. The illithids emerged somewhere and somewhen countless thousands of years ago, beyond the histories of many mortal races, and spread from one world to another, and another, and so on. It is explicitly stated in this book that the illithids appear in some of the most ancient histories of the most ancient races, even those that have no mention of other races.
Other evidence exists within The Sargonne Prophecies. The Illithiad described the Prophecies as misnamed, and that much of it sounds more like ancient myth than prophecy. Other translations take the name more literally, and states that The Sargonne Prophecies are in fact prophecy — or, perhaps more accurately, a history of the future.
However and whenever it occurred, when the illithids arrived in the Material Plane of the far past, they immediately began to build an empire by enslaving many sentient creatures. They were very successful, and soon their worlds-spanning empire became the largest one the multiverse had ever seen. Such was their might that the Blood War paused as the demons and devils considered a truce to deal with the illithid empire.
Eventually, the primary slave race of the illithids developed resistance to the mental powers of their masters, and revolted. Led by the warrior Gith, the rebellion spread to all the illithids' worlds, and the empire collapsed. The illithid race itself seemed doomed.
Fortunately for the illithids, Gith was betrayed by one of her own generals, Zerthimon, who believed she had grown tyrannical and over-aggressive. Civil war erupted, and the race factionalized into the githyanki and the githzerai. This disruption allowed the illithids to retreat to underground strongholds where they still dwell.
At some point, the illithids invaded the gith civilization from a neighboring plane of existence. Though the gith fought fiercely, they were no match for the psionic might of the mind flayers, and soon they were enslaved. Many were brought to the Outer Planes and elsewhere to serve as illithid slaves.
After Gith's rebellion, she led her people to the Astral Plane. While a few subject races and surviving illithids remained behind, the gith forerunners have departed the world, seemingly for good. If they retain any interest in the ruins of Zarum, it is well concealed. A portion of the ruins of Anithor were eventually colonized by the drow of House Kilsek, who named their new settlement Kalan-G'eld.

Basic Information

Anatomy

Illithids have humanoid bodies, with octopoid heads. They have four tentacles around a lamprey-like mouth, and require the brains of sentient creatures as part of their diet. An illithid who snares a living creature in all four of its tentacles can extract and devour its living brain.

Genetics and Reproduction

Illithids are hermaphroditic creatures who each spawn a mass of larvae two or three times in their life. The larvae resemble miniature illithid heads or four-tentacled tadpoles. Larvae are left to develop in the pool of the Elder Brain. The ones that survive after 10 years are inserted into the brain of a sapient creature. Hosts are determined in a very specific manner. Hosts generally are humanoid creatures that are between 5 feet 4 inches and 6 feet 2 inches. The most desirable of races for hosts are Humans, Drow, Elves, Githzerai, Githyanki , Grimlocks, Gnolls, Goblinoids, and Orcs. Upon being implanted (through the ear canal), the larva then grows and consumes the host's brain, absorbing the host's physical form entirely and becoming sapient itself, a physically mature (but mentally young) Illithid. This process is called ceremorphosis. Illithids often experiment with non-humanoid hosts, but ceremorphosis involving other creatures usually fails, killing both host and larva. The transformation between the host (almost always a human or similar humanoid, such as a elf or dwarf) takes about a week, unless detected and removed within about thirty minutes of injection into the incapacitated host.
When an Illithid undergoes ceremorphosis, it can occasionally take on some elements of the absorbed host creature's former mind, such as mannerisms. This typically manifests as a minor personality feature, such as a nervous habit or reaction (e.g., nail-biting or tapping one's foot), although the process that determines the type and number of traits so inherited appears to be stochastic. Some adult Illithids have even been known to hum a tune that its host knew in life. Usually, when a mind flayer inherits a trait like this, it keeps it a closely guarded secret, because, were its peers to learn of it, the Illithid in question would most likely be killed. This is due to an Illithid legend of a being called the "Adversary". The legend holds that, eventually, an Illithid larva that undergoes ceremorphosis will take on the host's personality and memory in its entirety. This Adversary would, mind and soul, still be the host, but with all the inherent abilities of an Illithid.
Occasionally, ceremorphosis can partially fail. Sometimes the larva does not contain enough chemicals to complete the process, sometimes there is psionic interference. Whatever the reason, it has happened that ceremorphosis has ended after the internal restructuring, resulting in a human body with an Illithid's brain, personality and digestive tract. These unfortunates must still consume brains, typically by cutting open heads (as they lack the requisite tentacles). These beings are often used as spies, where they easily blend in with their respective host types.

Ecology and Habitats

Illithids spend much of their time in intense study and experimentation, gathering knowledge of all sorts that will enable them to eventually reconquer the universe and hold it for good. They frequently meddle in the politics of other races through subtle psychic manipulation of key figures, not to cause chaos but so as to better understand the dynamics of civilization. They regularly probe the minds of surface dwellers so as to gather intelligence and learn about new advances in magic and technology. Illithids also do a good deal of research themselves, mainly focused on developing new psychic powers.
Illithids regularly conduct raids on all sentient settlements to acquire new thralls, because their existing stock of sentient thralls do not breed fast enough to satisfy their food and labor needs. Typically, a group of mind flayers will teleport to the settlement and swiftly incapacitate them with their psychic powers. The captives will then be marched all the way to the illithids' underground settlement by specially trained and conditioned thralls. Great care is taken to cover their tracks.

Additional Information

Perception and Sensory Capabilities

Illithid eyes are pale white, and though their vision is limited to roughly 60 feet, they can see perfectly well in both darkness and light.
Their sense of hearing is slightly poorer than a human's; they have difficulty distinguishing between several sounds mixed together.
Their skin is mauve and covered in mucus, and is very sensitive to sunlight; illithids who make extended forays to the surface must wear protective clothing to maintain their skin moisture.
Among the most feared illithid abilities is the dreaded mind blast, where the creature emits a cone-shaped psionic shockwave with its mind in order to incapacitate any creature for a short amount of time. Illithids also have other psionic powers, generally telepathic in nature, including abilities of a defensive psionic shield and powers of psionic domination for controlling the minds of others.

Civilization and Culture

Major Organizations

The Elder Concord Illithids are ruled by the dictates of the Elder Concord. The Elder Concord has the power to set community goals, arrange for the execution of these goals, arbitrate disputes between illithids, and take care of all other details important to the ruling of a community. The Elder Concord consists of senior illithid representatives from each of the recognized factions (translated better as Creeds) that exist within the community.
In its role as shepherd of the community, an elder brain is privy to every decision made by the Elder Concord. In addition, it has the power to propose courses of action for debate, place tie-breaking votes, and, most importantly, veto courses of action with which it does not agree. Every illithid community has an Elder Concord.
When the goals of more than one illithid community overlap, the two or more ruling concords in question form a temporary body referred to as a Grand Elder Concord. This body oversees the execution of the pertinent objective. The Grand Elder Concord remains viable as long as its primary goal remains unrealized. Once it achieves its objective, it dissolves.

Aedileators

Within its sphere of influence, the Elder Concord elects various illithid officials, called aedileators. Though not members of the Elder Concord, these officials report to that body and are responsible for running the day-to-day affairs of the community. Each particular aedileator has the power to draw excess thralls from the general thrall pool to accomplish its tasks. Individual aedileators are variously responsible for public order, repairing or building public works, and handling disputes between individual illithids-to name a just a few of their important roles

Quastors

The Elder Concord also elects officials, called quastors, that are responsible for a community's finances. Quastors hold and account for the funds available to an Elder Concord. Normally such funds stem from the dues each Creed must pay in order to remain a recognized faction. However, activities directed solely by the Elder Concord sometimes prove profitable as well. Specific Creeds also elect internal quastors to see to their individual finances.
One oft-recurring duty of a quastor involves the dispatchment of illithid merchants to various underdark communities (not necessarily confined only to other illithid communities). These merchants trade for specialized merchandise and rare materials. In fact, sages speculate that certain less-than-scrupulous thrall races engage in an illicit brain trade.

Vigileators

Vigileators are those officers chosen by the Concord to direct the security of an illithid community. Illithid security forces normally consist of three bodies, each of which is responsible for a particular security concern: internal order, military offense and defense, and intelligence. A single vigileator heads each security branch.
Illithids elected to fill a particular official position must put aside any Creed affiliations for the good of the community as a whole. Any illithid demonstrating a Creed-based bias in the execution of its duties loses its position and standing within the community immediately. Of course, Creeds can legitimately attempt to influence an official's performance through its representative on the Elder Concord.
Creeds whose ideologies most closely resemble the thrust of the Generally speaking, elected officials are chosen from particular duties in question.

Major Language Groups and Dialects

Illithids typically communicate through psychic means. They project thoughts and feelings to each other in a way non-illithids can scarcely comprehend. When they do feel the need to write, illithids do so in qualith. Instead of typical alphabet-based writing, illithids write in qualith by making marks consisting of four broken lines. They use each tentacle to feel the breaks in the lines, making it basically similar to braille. However, qualith is extremely complex, as each line modifies the preceding lines through explaining abstract concepts associated with the above words in ways no human can understand; only by understanding all four lines simultaneously can the meaning be understood properly.

Common Taboos

Illithid society also maintains a long-standing taboo related to deviations to or failures of the ceremorphosis process and hunt and destroy such exceptions. Occasionally mind flayer communities are attacked (often by vengeful Githyanki and Githzerai) and their inhabitants must flee. This leaves the larvae unattended. Bereft of exterior nourishment, they begin to consume one another. The survivor will eventually leave the pool in search of food (brains). This unmorphed larvae is known as Neothelids. If the Neothelid consumes an intelligent creature it will awaken to sapience and psionic abilities and grow to immense size, while retaining its memories of savage survival. In Complete Psionic, it was revealed that Illithids have a step between larva and Neothelid called a Larval Flayer, which looks like an overgrown tadpole. The existence of these beasts is a guarded secret among Illithids, and it is considered impolite to speak of them.

Common Myths and Legends

When an illithid undergoes ceremorphosis, it can occasionally take on some elements of the absorbed host creature's former mind, such as mannerisms. This typically manifests as a nervous habit and/or reaction, like nail-biting or tapping one's foot. An adult illithid has even been known to hum a tune that its host knew in life. When a mind flayer inherits such a trait, it usually keeps it a closely guarded secret; were its peers to learn of such a trait, the illithid in question would surely be killed. This is due to an illithid legend of a being called the "Adversary." The legend holds that an illithid larva that undergoes ceremorphosis will eventually take on the host's personality and memory in its entirety. This "Adversary" would, mind and soul, still be the host, but with all the inherent abilities of an illithid.
In the 2nd Edition AD&D adventure Dawn of the Overmind, author Bruce Cordell revealed the nature of the Adversary. This being was an illithid created from the host body of a man named Strom Wakeman, a character referenced in some of Cordell's other 2nd Edition works, most notably as the "author" of The Illithiad supplement.
Wakeman, an enterprising trader and scholar of Underdark exotica, allowed himself to be captured by the illithids on one of his expeditions. Through the use of a non-magical mixture of various herbs that Wakeman named laethen, he was able to preserve his consciousness through ceremorphosis, and learned to use his psionic powers to keep from having to consume brains. Thus he worked against the illithid plots from within. The legend of the Adversary was born from his frequent sabotage, though the actual acts were never connected to him. The players' characters in the adventure become his agents in stopping the illithids' plans, as his own movements rely upon secrecy from his "fellow" illithids.
For another person to imitate Wakeman's deed would require at least one dose of laethen (the making of which Wakeman kept secret) and to be put under ceremorphosis within a week of consumption. The drug only has a 40% chance of success, and the new illithid must also never consume a brain, for the act destroys the host's personality and replaces it with the illithid personality.

Interspecies Relations and Assumptions

Currently, the illithids are in a period of intense study and experimentation, gathering knowledge of all sorts that will enable them to eventually reconquer the universe and hold it for good. They frequently meddle in the politics of other races through subtle psychic manipulation of key figures, not to cause chaos but so as to better understand the dynamics of civilization. They regularly probe the minds of surface dwellers so as to gather intelligence and learn about new advances in magic and technology. They also do a good deal of research themselves, mainly focused on developing new psychic powers.
Illithids seek to rebuild their former empire wherein all other species were their slaves, so they view any sentient creature as worthy only of being their slaves or their food. They are pragmatic, however, and will trade with other races, such as dark elves and gray dwarves, who are too strong to be conquered. They also trade with the Neogi in order to obtain slaves.
Illithids regularly conduct raids on all sentient settlements to acquire new thralls, because their existing stock of sentient thralls do not breed fast enough to satisfy their food and labor needs. Typically, a group of mind flayers will teleport to the settlement and swiftly incapacitate them with their psychic powers. The captives will then be marched all the way to the illithids' underground settlement by specially trained and conditioned thralls. Great care is taken to cover their tracks.
Their archenemies are the githyanki and the githzerai, descendants of the rebellious slaves who destroyed their empire millennia ago. Hunting and slaying illithids whenever they can is an integral part of their cultures.
Illithids fear the undead because these creatures, even the sentient ones, are immune to telepathic detection and manipulation, and have no brains to consume. Confronting such mindless creatures can even be traumatizing to some of them.
Supposedly, Illithids are one of the few races respected by the aboleths. This is because the aboleths remember the origin of almost every other race, through their hereditary memory. However, illithids, as far the aboleths can remember, just appeared without preamble, which scares them.

The Illithid Threat (Guardians of the Fontier)
Illithid (Academic)
Illithid (FR Wiki)
Illithid (Grey Wiki)
Illithid (Statemaster)