How is it that all those complicated, counterweighted poison dart traps can still work perfectly in a crypt after a thousand years? Simple: A gearghost was maintaining them the whole time.
You might think I’m being cheeky, but I’m actually all about the gearghost. I like in-game solutions to out-of-game/suspension-of-disbelief-type problems (Exhibit A: my love of Ghostwalk). And it makes sense that a frustrated thief’s post-mortem existence could be just as unquiet as any fallen paladin or jilted lover (particularly in a game world like Golarion with deities like Norgorber and Zyphus roaming around).
If anything, I think we can expand our notion of where we find gearghosts. I think they might pop up in any number of locations—any place where accidents happen too often or too many cogs and flywheels are involved. There’s also something uncanny and almost alive about certain kinds of machinery…and in a fantasy world where elemental spirits power golems and undead servitors are more reliable than electricity, many machines may actually be (at least a kind of) alive. When they break down or are discarded, where do those spirits go? More than a few might become gearghosts.
So rather than provide adventure seeds for gearghosts in trap-filled dungeons—I figure you’ve got that on lock—here are some undead that might be appearing at a local golemworks near you.
A strange mill features a water wheel that turns and turns despite the absence of any river. The mill actually harvests energy from a miniature vortex to the Elemental Plane of Air. The mage who constructed the mill persists as a gearghost, and he has rigged traps all over the structure to prevent his life’s work from being tampered with. If the gearghost is destroyed—even temporarily—the party’s troubles are not over. The wheel ceases turning, and the obstruction causes two irate large air elementals to manifest and attack anyone they see.
A trapmaker dies in his own home after gremlins sabotage his handiwork. Rather than be bound to his workshop, he is instead bound to the fey mob. Every machine they destroy, he reconfigures into a trap. To defeat his rejuvenation effect, all the gremlins must be slain and then anointed with holy water in the area of a hallow spell (as per the Bestiary 4). Otherwise the gearghost will return with the gremlins in tow as zombie slaves.
Tinkerer Hamden Rhodes put too much of himself into his clockwork creations—for some of them grew to have souls. When after decades of faithful service he junks the very first clockwork servant he ever built, the construct’s spirit returns in wounded outrage. It wants Hamden’s corpse sent to the scrap yard, but not before terrorizing him and everyone in the vicinity with a series of deathtraps.
—Pathfinder Bestiary 4 123