We recently discussed importing sci-fi-esque invasion scenarios into fantasy plots. Few monsters work better for this than intellect devourers. Being telepathic, brain-eating, body-snatching monsters makes them bad enough, but add in at-will invisibility and inflict serious wounds effects and you have some serious foes. Eventually though, the PCs should be treated to the spectacle of a devourer claiming a host—the sight of a clawed brain possessing a helpless NPC (or PC) by crawling into its mouth should alarm the players quite nicely.
Parties in the city of New Flourint are famously decadent, yet no successful gathering begins without a spellcaster blessing the host and the guests of honor with protection from evil spells. Four social seasons ago an intellect devourer wore the city’s best and brightest like a dandy going through new suits. Though the body count stopped climbing, the beast was never caught, and the pall it cast still hangs over the formerly libertine aristocrats. If alive, it likely dwells in the chambers just outside the city walls, where Old Flourint lies buried in lava.
Cave elves, dwarves, hobgoblins, and goblins alike congregate in Rift City—a series of terraced mushroom fields, simple neighborhoods, thermal baths, and rope bridges buried deep in a canyon. The people are reserved but welcoming to travelers, especially those bearing exotic fabrics, foods, and spices, though they avoid questions about how such disparate races came to live in peace. The only things that keep the city from being a veritable paradise are the gremlin pests who raid the mushroom fields and dismantle the city’s engineering. But if questioned rather than exterminated, the gremlins adamantly maintain they are just trying to drive “the bad ones” out.
On an isolated mountain peninsula, a tribe of intellect devourers has held sway since their arrival in the heart of a meteor. Posing as emissaries from the stars, the telepathic devourers are slowly working their way through the hierarchy of the nation’s lamas. Two factors have aided their dominance—their original discovery by a deeply religious people already used to sending their elders and second sons to serve the monks, and a permanent gentle repose effect (the result of a planar conjunction) that has allowed the aberrations to inhabit their hosts for as long as six months, rather than a mere week. But with the coming of a comet in the southern sky, the land falls out of phase with the Sphere of Harmony and the intellect devourers begin burning through their Most Honored Hosts.
—Pathfinder Bestiary 180
Intellect devourers are definitely a relic from another age of gaming (Eldritch Wizardry, in fact)—another one of those monsters you either love (“It eats brains and possesses bodies!”) or hate (“It’s a brain crossed with a Schnauzer!”), and semi-psionic to boot. You can tell the Paizo folk love them though—even before the Pathfinder Core Rulebook came out, intellect devourers got their own nicely creepy domain in Into the Darklands.