Idea totally stolen from Steph Cherrywell's delicious comic Intragalactic. No, really, it tastes good - lick your monitor and see.
The Brainworm, Large (Also know as the 'Brainworm - Large,' the 'Brainworm (large)' or the 'Brainworm fingerquotes Large fingerquotes') is a... um, worm that eats brains, absorbing the information contained therein. It then uses the corpse to find more prey... unless the victim was sentient.
The parasite survives by gripping onto the skull of its victim via four strong mandibles, gnawing into the brain cavity and devouring the contents. The worm then "plugs into" the sensory organs, takes control of the muscle system and receives sustenance through the victims' usual methods. It can live this way indefinitely, preserving the corpse it's riding in an (almost) perfect state. However, the worm can't progress in its growth cycle this way, and usually uses the host to reach a fresh meal of brains.
However, when a brainworm mistakenly latches on to a fully sentient lifeform, something odd occurs. While the victim still dies, the memories and emotions absorbed by the brainworm form a dominant personality. The worm both recalls being a brainworm, and being the victim. In this state, the parasite effectively becomes a new incarnation of the dead host and will simply try and live the life it technically ended. Often with a massive guilt-complex.
For crunchiness (rules, for you non-DnD blog readers), most mental stats of a brainworm-bitten PC or NPC remain the same. The primary difference is that the actual life form is the large grub-like worm-thing stick out of their head, and only damage to it can kill the character. The corpse can suffer damage, and at 0 - CON hitpoints the worm will be forced to detach from its ruined host. Otherwise the corpse can go indefinitely. No critical hits or sneak attack bonuses. The only drawback is that the body cannot heal naturally, and potions of healing (as with any other potion or poison) only affect the worm. Healing spells work normally.
If someone aims for the worm, use the worm's AC to determine if they hit it. If attacking the worm specifically, sneak-attack damage, critical hits DO apply.
If someone attacks the character without aiming for the worm (most figure it's a hat), natural 20s are assumed to hit the worm, but still without bonus damage. Things that hit everything in an area, like fireballs, hit the worm.
So, here's what happens when your PC's brain gets eaten. Drop your Charisma 1d4 for the giant bug and the fact you look like death. Raise your Intelligence and Wisdom each a point for the addition of the brainworm's past meals (which would have all been non-sentient things, remember?). Raise your Constitution 1d4 for being dead meat. Lower your Dexterity a point for mild joint stiffness and counterbalancing the big thing sticking out of your head.
And without further adieu, the Brainworm:
Attached Brainworm, Large
Abberation, Tiny (although quite Large for a worm)
AC: The host's, minus armor, plus 5 (+2 size, +3 natural armor)
Saves: Fort + 1, the other two are the same as the host's
Abilities: Str 18, Dex Host's -1, Con 13, Int Host's +1, Wis Host's +1, Cha Host's -1
Unattached Brainworm, Large
Speed: 10 ft
AC: 15 (+2 size, +3 natural armor)
Saves: Fort +1, Reflex +0, Will +1
Abilities: Str 18, Dex 11, Con 13, Int 2, Wis 11, Cha 3
Attacks: Leap and Grab with Mandibles +5
Special: After a successful grab, the worm can automatically bite for 1d10 damage each round until dislodged. On a natural 20 it's grabbed the opponent's head, it may spend a turn to maneuver to the back instead of biting. On the next round, it instantly opens the skull and devours the brain, killing the victim. On the subsequent round it may take over the corpse.