What book do I find cerebral parasites in? I've got an adventure that has them in it but doesn't give me any of the details of what they do. The adventure came from an issue of Dragon magazine, in an adventure on Baba Yaga. It says that every time a psionic power is used, they attract 2-20 cerebral parasites to the user per round.
Cerebral parasites can be found in the Expanded Psionics Handbook but also in the Monster Manual (1977) for Advanced Dungeons & Dragons
"The Dancing Hut" by Roger Moore for Advanced Dungeons & Dragons that's from Dragon #83 on Psionic Alterations says
Baba Yaga's Hut produces alterations in psionic powers as well as magical ones. And, to make matters worse, the Hut attracts cerebral parasites as a side effect of its planar travel capabilities. Any psionic character within 120' of the Hut who uses a psionic talent (attack/defense mode or discipline) will immediately attract 2–20 parasites to him per round. Any method used to get rid of the parasites will last only one round, and more will come after that. (Assume there are hundreds hovering around the Hut at any time, as well as dozens within each area inside it.) (38 and emphasis mine)
This AD&D adventure seems to be the source of your question. (The Advanced Dungeons & Dragons, 2nd Edition adventure The Dancing Hut of Baba Yaga (1995) appears to lack mention of cerebral parasites. Further, neither this Dungeon magazine index nor any other Dungeon magazine index I found (and there are several) lists a Dungeon magazine version of a Baba Yaga's Hut-based adventure. Not even EN World's conversion library has a Third or 3.5 entry for the adventure.)
Anyway, in Advanced Dungeons & Dragons, the cerebral parasite was a for-reals creature, a description of which is in that edition's Monster Manual (14). To summarize, the cerebral parasite is a microscopic creature that feeds off psionic power points whenever a psion uses a psionic ability, and a cerebral parasite that eats enough psionic power points produces another cerebral parasite—and so on—until the DM laughs and tells you, "Your psionics don't work," and the cleric casts cure disease to kill them all. (Cerebral parasites not DMs.)
As this fine answer and this fine answer mention, the Expanded Psionics Handbook for Dungeons & Dragons 3.5 turned the cerebral parasite from a monster into a disease: cerebral parasites (68). This change, obviously, makes that 2–20 figure in need of adaptation.
Further, because psionics—including psionic diseases—are already part of the SRD, it's unlikely any publisher saw fit to give these microorganisms a proper stat block. (Just to be sure, I searched Drive-thru RPG for "cerebral parasite" and got one hit for a Second Edition Dark Sun adventure, and that leads me to believe that there were no other cerebral parasites entries for any other games but AD&D, Second Edition, and the disease version from Third Edition.)
Thus if you're adapting Moore's adventure to Dungeons & Dragons 3.5, you'll need to figure out what sort of adaptation is fair. That is, this DM doesn't recommend simply exposing the psion to the disease cerebral parasites 2d10 times after each time the psion manifests a power—the Fort save DC (15) is too low and its damage too little (1d8 power points) for it to matter to high-level PCs like those who'd take on an adventure like "The Dancing Hut." (Moore recommends the adventure for level 10 and higher AD&D characters, equivalent to Dungeons & Dragons 3.5 characters of level 15 or higher!)
As a DM who's adapted AD&D adventures to D&D 3.5 and who has DMed for high-level PCs that employ psionics, I can offer the following unplaytested suggestion that I've renamed to avoid confusion.
Each time after a creature manifests a psionic power in the Hut, 2d10 almost undetectable power parasites become attached to his aura. When a creature that has power parasites attached to his aura manifests a power, that power manifests normally except that an additional number of power points are expended equal to the number of power parasites attached to the psionic creature's aura. (This can't cause a psionic creature's power points to fall below 0). A remove disease effect rids a psionic creature of all power parasites.
A creature that takes 1 hour examining a creature with power parasites attached to his aura and that succeeds on a Heal skill check (DC 40) or a Knowledge (psionics) skill check (DC 30) detects the power parasites and knows how to get rid of them.
In play, this would mean after a psion manifests a power, the DM would roll 2d10 and note the total. Then, the next time the psion manifests a power, the DM would say, "Also subtract an additional X power points," where X is the first result; then the DM rolls 2d10 again and adds that new result to X. And so on. It would be up to the PCs to spend the time and have the skills to identify the cause then be up to the PCs to rid the psion of it. The first time the power parasites drain some power points, the PCs will probably shrug it off, but by the fourth they should be scrambling, having lost a total of 12d10 power points!
This version isn't as old-school-nasty as the actual cerebral parasite—its reproduction method pretty much guarantees that a party without a cleric will soon never uses psionics again—, but it may be enough that a psion without an infinite recharge trick will soon worry, and, if it's not, adapt it to fit so it is. (Note that it's easier for a Third Edition party without a cleric to have access still to a remove disease effect than it was for an AD&D party!)
They're in the SRD:
Disease, Cerebral Parasites
Spread by contact with infected psionic creatures; contact; DC 15; incubation 1d4 days; damage 1d8 power points.
Cerebral parasites are tiny organisms, undetectable to normal sight. An afflicted character may not even know he carries the parasites—until he discovers he has fewer power points for the day than expected. Psionic creatures with cerebral parasites are limited to using each of their known powers only once per day (instead of freely manifesting them). See the note about diseases under Cascade Flu, above.
The mentioned note:
As with any disease, a psionic character who is injured or attacked by a creature carrying a disease or parasite, or who otherwise has contact with contaminated material, must make an immediate Fortitude save. On a success, the disease fails to gain a foothold. On a failure, the character takes damage (or incurs the specified effect) after the incubation period. Once per day afterward, the afflicted character must make a successful Fortitude save to avoid repeating the damage. Two successful saving throws in a row indicate she has fought off the disease.
I don't own the book to check, but I'd assume they're from Expanded Psionics Handbook, that being where the majority of 3.5e psionics content comes from.
This would show that whatever character attracts these parasites would go through the standard disease rules 2-20 times per round. If they're an NPC, this would likely not even affect the adventure. This seems somewhat strange. I'd assume that they're meant to be passed to other PCs, which would inhibit their psionic powers. Since it seems like only psionic creatures can contract it, you can likely just ignore it if you don't have any. If you do, follow the rules in the note.
Expanded Psionics Handbook, p.68
It is found under the Psionic Maladies section. It is listed as, and has stats as, a disease, rather than a creature.
Disease, Cerebral Parasites: Spread by contact with infected psionic creatures; contact; DC 15; incubation 1d4 days; damage 1d8 power points.