Intellect Devourers

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Biology

Intellect devourers are small animals, the size of cats.  They walk on little cat legs.  They have been compared to "brains on four legs" because of their rugose appearance.  Pure water is noxious to them; they dwell most happily in ammonia.

They have eyes on the sides of their bodies, hidden among the wrinkles.  The eyes look remarkably like human eyes, but are not usually seen.  Golden eyes like royal jelly, wet and proud.

Intellect devourers can unravel themselves.  It looks like braids of wet, pink yarn.  They unravel themselves and then they slither up your nose.  Then they use their little mouse claws to shovel your brains, which they eject out of your nose, like a dog digging in a flower bush (and usually leaving a telltale pulp on the floor).

Then, having replaced most of your brain and some of your spinal cord, the intellect devourer puppets your body around like a goddam skin car.  Enough brain matter is retained for the intellect devourer is "know" some basic facts of the puppet's life: name, language, basic history, close friends.

The intellect devourer's body--which is a strange type of protoneural tissue--gradually turns into real neurons.  After a week, the intellect devourer becomes irrevocably wedded to it's host.

The devourer then has a decision to make.  It can leave the body before the 7 days are up, thereby creating a brainless corpse.  The intellect devourer retains some of the host's memories, and permanently gains a point of Intellect.

Alternatively, the intellect devourer could remain in the host.  If this happens, the intellect devourer eventually becomes virtually indistinguishable from a normal biological human, and actually loses the ability to leave the host.  The intellect devourer basically replaces a human brain, in form, function, and composition.  The only difference is that they have an alien mind (though they'll pretend differently) and a brain that is a rich purple color.

Psychology

They are fully intelligent and communicate telepathically among themselves.  Their word for themselves is phanikin.

According to their own myths, they were the original sentience in the universe.  Previously, there were just mindless animals eating and shitting everywhere until they stepped in and started thinking the first thoughts.  They were the first ones to start summing cogito and ergo.  They created the first people when they got trapped inside animals.

When an intellect devourer takes over a body, temporarily or permanently, much of the brain is preserved.  Elements of personality and memory persist, and the intellect devourer is not entirely unaffected by them.  They are shaped by the minds that they pass through, by the people they have been.

An intellect devourer that takes over a loving mother will hesitate to kill their host's baby.  (Most likely, they'll drop it off at the nearest orphanage.)  A devourer that has just replaced 75% of its host's brain may find itself developing strange feelings of honor and devotion.

Once an intellect devourer has eaten 75% of your brain, you are 75% dead.  Or to put it another way, you are 25% alive.

Culture

In the Vaults of Voyona, a city is populated entirely by intellect devourers.  They will pay high prices for interesting slave bodies to possess.

In the Stranglium Abysm, the intellect devourers possess only derro.  This has made them weird and distrustful of skulls (which is where conspiracies come from).  They shun taking over brains, and instead grow huge and weird, becoming Giant Mutant Intellect Devourers.

In the city of Gren, intellect devourers have formed a partnership with law enforcement.  Those who would otherwise be condemned to death are instead given to the devourers.  Once they possess their new bodies, they join the city guard in order to hunt down more wrong doers.  (Some say that they are overzealous in this task.)  Almost half of the city guard is now controlled by ex-intellect devourers.

In the Unspun Reaches, a monastery of monks works alongside a monastery of intellect devourers.  After passing through the minds of nine monks, an intellect devourer reaches enlightenment inside the skull of the tenth monk.  Together, the resultant creature (which contains small elements of all of the 10 devoured monks) is said to be a powerful force of balance and peace in the world.  The shared basement is infested with leaping hoards of young intellect devourers.

In the city of Yog (the greatest city of the Darklands) there are several ethnic groups of intellect devourers present.  The largest of these are the Barabi Burzum, who are deeply religious (see Psychology).  They believe that possession by intellect devourer is the natural step of a creature's life.  (I.e. childhood --> puberty --> adulthood --> devourerhood.)  They believe that they become the humanoid that they take over.  And so an intellect devourer that takes over a farmer will then spend a great deal of time and effort convincing everyone that he is the same farmer with the same goals.  They feel responsible for their host's children and spouses, etc.

In the city of Yog, another group of intellect devourers are the Jungerani.  Like the Burzum, they are deeply religious.  They believe that mind-control is abhorrent, and so they hunt abominations such as mind-flayers, wizards, and other intellect devourers.  (An intellect devourer can empty a skull in a moment regardless of whether that skull holds a human brain or another devourer.)  They care nothing for humanity (the "least race") who they see as chattel.  They sometimes use brutal measures in their efforts to rid the world of mind-control.

In the city of Yog, the intellect devourers sell their own young (traditionally locked inside a gilded cage shaped like a head) to adventurers.  The idea is that an adventurer treats a juvenile intellect devourer generously while it is in its cage, then when a foe is captured, they are given to the intellect devourer to inhabit.  The adventurer gains an ally, the young devourer gets a host, and the baby-selling devourer ensures the success of its progeny.  Everyone wins!

Stats

Intellect Devourer
HD 1 AC chain Claw 1d3
Move human Int 8 Mor 5
*Devour Intellect: At-will.  Target within 50' takes 1d8 Int damage, save for half.  If a target takes 8 damage from this attack, the intellect devourer learns one of the target's secrets.  Unlike most ability score damage, this Int damage recovers at a rate of 1 point per hour.
*An intellect devourer can crawl in the nose of a helpless target, excavate their brain, and take over their body.  This takes 1 round, during which the intellect devourer is helpless.  If they remain for more than 7 days, they lose their special abilities (including the ability to leave the skull).
*If you are immune to ESP, or if the devourer is not aware of you, it is much less effective at dodging your attacks (treat its AC as unarmored).  This creature is automatically aware of all creatures in 50' that have Int 10 or greater.

Giant Mutant Intellect Devourer
HD 7 AC chain Claw 1d6
Move human Int 14 Mor 
*Zone of Idiocy: All creatures within 20' are reducing to drooling, non-verbal idiocy.  Players (not characters) under this effect cannot speak, and must grunt/mime out their character's actions.  Under this effect, complex plans are impossible.  Basically, if you can't make yourself understood in a few seconds of grunting and miming, your character can't do it.  Spells are still possible.  It's also possible to declare a complex action outside of the zone ("I'm going to run up and rub salt in its eyes.") and then run in and do it.  (The Zone of Idiocy doesn't make you forget plans you invented outside of it.)  No save
*Devour Intellect: At-will.  All targets within a 50' cone take 1d8 Int damage, save for half.  If a target takes 8 damage from this attack, the intellect devourer learns one of your secrets (usually the most plot-relevant one).  Unlike most ability score damage, this Int damage recovers at a rate of 1 point per hour.
*If you are immune to ESP, or if the devourer is not aware of you, it is much less effective at dodging your attacks (treat its AC as unarmored).  This creature is automatically aware of all creatures in 50' that have Int 10 or greater.

Usage Tips

Intellect devourers work great in small packs.  They're smart enough to use clever tactics (sometimes).  They can be quite deadly if a large number of them attack simultaneously and focus fire on a single character.  But then, that's true of all sorts of monsters.  They "bark" psychically.  Just telepathic barks and snarls.

It's possible for an intellect devourer to try to slither up someone's nose while they're sleeping, but the person will wake up halfway through, and they'll have a single turn to deal with a (weak, slow) purple pulp snake trying to crawl up their nose.

Intellect devourers could also work as a mid-fight complication.  Oh sure, you killed all 5 of the orcs, but now there are these horrible little monsters crawling out of the orcs' piggy nostrils.  (Bear in mind that intellect devourers pretty much only attack Int, which is a big help if any of the PCs were left at low HP after fighting orcs.)

Since intellect devourers retain scraps of their meal's memories, its also possible to base a quest about retrieving some datum from an intellect devourer.  The beastie in question could be located in a hive of the damn things, or. . . just about anyone, really.

They work well as add-ons to a fight.  They're low-value targets (you'd still want to kill the mind flayer first) but if you have any PCs helpless and unconscious, the intellect devourers can suddenly become a lot more threatening.

It's really fun to roleplay massive Int loss.  As long as the party has at least one party member with normal Int to babysit everyone, the game goes on mostly the same as it always does.  The Int 3 fighter can still kick down downs, the Int 3 wizard can still cast any prepared spell (at least in my game).

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