It almost feels silly to offer adventure seeds to go with ghosts—there are more than enough ghost stories out there, not to mentions F. Wesley Schneider’s excellent entry in Classic Horrors Revisited. (GMs who like to trick out their monsters with different special abilities will definitely want to check out CHR’s new corruptions; meanwhile, 3.5 fans will want to check out Libris Mortis.) The takeaway point, as the Bestiary notes, is that ghosts don’t arise by sheer chance. Behind every manifestation, there is always a “Why.”
In the wilderness, ghosts are especially prevalent near quicksand, bogs, tar pits, and other death traps—the slow horror of the deaths, combined with the urgency that drove the victims so far from civilization in the first place, make for fertile grounds. The quest to quiet a tengu assassin reveals not only the sinkhole that claimed him but also the strange roper kingdom beneath. The rubbery things still chortle over the message of regret carved into his scabbard.
The ghost of a young girl torments a boy’s boarding school. Identifying her won’t be easy, since she’s actually a he. A gifted cartwright’s son and scholarship student, Athem Lars was loathed by his wealthier, less talented peers. They pooled their allowances for a cursed belt that changed Athem’s gender…and he hung himself with it that very night.
The city-state of Sebastian is practically a theocracy. Yet the east wing of its cathedral cannot be used, thanks to the presence of a pair of ghosts that have resisted all attempts at exorcism. The key to laying to rest the two star-crossed lovers—an aasimar and a tiefling—is for an identical couple to wed in their place. But convincing the prelates of this remedy will be no easy task, in a land where tieflings are stopped at the border by law and killed at birth by custom.
—Pathfinder Bestiary 144–145
Speaking of ghosts, if you have not done so, you must check out Ghostwalk by Monte Cook and Sean K. Reynolds. I don’t think there is a book in the 3.0/3.5 canon that arrived with such little fanfare and was packed with such amazing nuggets. Read it.