Ceremorphosis was a bodily change that occurred when an illithid tadpole reached maturity and was inserted into the brain of another being, usually a human. The tadpole ate away the victim's brain matter and essentially replaced the brain, erasing all of the subject's personality and memory, but leaving the physical body alive and under the tadpole's control. After this, morphological transformations occurred and after a week a new illithid was created.
It was common for the newly formed illithid to retain a few memories of its former host. Those memories were merely vestigial and rarely affected the new individual's personality.
Upon securing a suitable host, mind flayers began the procedure by rendering the candidate docile with a mind blast. A mature tadpole, selected after surviving a period of one decade in the elder brain's brine pool, was then inserted in the host's cranium through a nostril, ear canal, or eye socket. It then burrowed into the brain and quickly devoured the gray matter, growing in the process as it replaced the brain with its own tissue and attached itself to the brain stem like a parasite. This initial process, which happened over a period of a few hours, effectively completely replaced the victim's personality with the tadpole's, while still keeping the body alive for the second stage of the transformation.
It was only possible to interrupt ceremorphosis and save the host before this initial stage was completed, and even so it was only possible to do so by killing the tadpole, which was complicated by its location. The safest way was to incinerate or crush the host's head and then use spells such as resurrection, or true resurrection. Destroyed parts of the victim's personality could then be reconstructed via restoration and heal spells, as long as the damage was not complete.
After the initial stage was completed, the original creature was lost and beyond help, except for a miracle. Over a period of approximately one week, the host's body underwent extensive morphological and tissue changes, completely transforming into a mind flayer. Internal organs were reshaped, but retained their original purpose. Until ceremorphosis was completed, the emerging mind flayer remained helpless.
The newly formed mind flayer, despite being fully grown, was equivalent to an infant, and had to undergo a period of instruction and experience that lasted approximately twenty years, usually within the confines of a secure illithid city.
The physiological changes caused by ceremorphosis played a key role in the illithids' dietary habits. Since the tadpole acted more like a parasite of its host's body than a proper brain, it did not produce the enzymes and psychic energy necessary to keep it alive. These essential nutrients needed to be acquired through consumed brains for the body to survive.
The success of ceremorphosis was only guaranteed with a few humanoid races. In particular, only humans, elves, drow, githyanki, githzerai, grimlocks, gnolls, human-sized goblinoids, and orcs produced viable mind flayers. In addition, the candidate's height and weight also played a role in the process, since body mass was a factor in the tadpole's growth rate. Only humanoids weighing between 130 to 270 pounds (59 to 120 kilograms) and with heights between 5 feet and 4 inches to 6 feet and 2 inches (1.6 to 1.9 meters) were considered acceptable. Creatures outside that range caused the tadpole's evolution to proceed either too fast or too slowly, and both cases resulted in both the host's and the tadpole's death.
Some creatures were always considered unacceptable for ceremorphosis. Those included centaurs, derro, duergar, dwarves, giants, halflings, and kuo-toa. Any attempt to conduct ceremorphosis in one of those races invariably resulted in death for the host and the tadpole. Most non-mammalian species were also considered incompatible in principle.
While gnomes were also normally considered unacceptable candidates to ceremorphosis, it was possible to warp the process in order to produce a viable ceremorph, but the degree of success varied.
Since the height and weight ranges were a more determinant factor than species in the success of ceremorphosis, in principle even other races that fell within those ranges could still be compatible. For that reason, mind flayers constantly conducted experiments with newly found creatures.
A small variety of species could successfully undergo ceremorphosis, but if not one of the fully compatible races, the result was called a "half-illithid," also known as a "ceremorph" or "flayer-kin". They did, however, still possess most of the powers of a regular illithid. Examples of creatures that had undergone ceremorphosis successfully included:
- Brainstealer dragon
- The result of implanting a tadpole in a captured dragon, they sometimes ruled illithid settlements that did not have an elder brain.
- Gnome ceremorph
- By tampering with the normal ceremorphosis process, it was possible to produce a ceremorph by implanting a tadpole into a gnome. If successful, the new creature retained parts of its old personality and some of its memories. If the attempt was unsuccessful, a deformed creature known as a gnome squidling resulted.
- The result of implanting a tadpole in a captured beholder, these creatures were used as telepathic amplifiers to improve communication within the colony.
- The result of a tadpole being inserted into a deep gnome and then subjected to psychic surgery that channeled energy from the Shadowfell.
- The result of illithid tadpoles being inserted into lizardfolk. They were used as personal guards and slave laborers.
- Sometimes known as a slime chuul, it resulted from a chuul being implanted with an illithid tadpole.
- Created to serve as guardians for a mind flayer community. An urophion was a roper that had survived the tadpole implantation process. They were usually treated as second-class citizens but were permitted to join with the elder brain upon death.
- An illithid/yuan-ti hybrid. They had viper heads and scaly tentacles. Located in Ss'zuraass'nee.
Many other creatures, such as greater doppelgangers, were subject to becoming half-illithids but did not necessarily have unique names as in the preceding examples. Such transformations tended to involve the creature developing psionic abilities, sprouting illithid-like tentacles—assuming the creature did not already have tentacles—and acquiring a taste for eating brains.
Some sages believed that ceremorphosis had been a late development in the evolution of the mind flayer species. Evidence of this was found in the shape of their two-toed webbed feet, which suggested a pre-ceremorphic race of water dwellers.
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Gnome ceremorph • Gnome squidling • Mindwitness • Mozgriken • Tzakandi • Uchuulon • Urophion
Brain golem • Cranium rat • Nyraala golem • Illithidae • Illithocyte • Intellect devourer • Mind worm • Neothelid • Nerve swimmer • Oblex • Oortling • Ustilagor