
Tim Emrick |

Here's the relevant text from the GameMastery Guide:
The distinction between a trap and an undead creature blurs when you introduce a haunt—a hazardous region created by unquiet spirits that react violently to the presence of the living. The exact conditions that cause a haunt to manifest vary from case to case—but haunts always arise from a source of terrific mental or physical anguish endured by living, tormented creatures. A single, source of suffering can create multiple haunts, or multiple sources could consolidate into a single haunt. The relative power of the source has little bearing on the strength of the resulting haunt—it’s the magnitude of the suffering or despair that created the haunt that decides its power. Often, undead inhabit regions infested with haunts—it’s even possible for a person who dies to rise as a ghost (or other undead) and trigger the creation of numerous haunts. A haunt infuses a specific area, and often multiple haunted areas exist within a single structure. The classic haunted house isn’t a single haunt, but usually a dozen or more haunted areas spread throughout the structure.
My interpretation is that a haunt is powerful psychic or spiritual impression (or "echo") left by a traumatic event. It has some of the characteristics of an undead creature, but lacks the substance, identity, will, or whatever to be a creature in its own right. Sometimes, the haunt has a limited ability to communicate with the living, but that's usually more like a broken record of the original victims' anguish than any kind of true dialogue.
This sort of "psychic echo" to frequently used by real-word paranormal researchers to explain things like cold spots, poltergeist activity, and the sort of apparitions that are always seen repeating certain actions but that can't really interact with the living.
Just last night I ran a PFS scenario involving a number of haunts, with very flavorful--and nasty!--examples designed to both pose a serious threat to the PCs and to reveal the dire history of the place. Which, to be honest, is the best way to use haunts.