http://www.d20pfsrd.com/gamemastering/haunts
Party enters area of a haunt. Suprise round starts, haunt acts on 10, PCs must make perception check according to haunt discreption, otherwise cannot act in suprise round.
PCs acting can damage the haunt with positie energy or with attacks that match weaknesses or can exploit weaknesses otherwise (e.g. cast a hide from undead in case the haunt has that weakness). No other ways to effect the haunt exists (e.g. greater globe of invulnerability is useless)
If haunt is not reduced to 0 HP before ini 10 haunt effects PCs in radius and not protected.
Further PCs acting after ini 10 can damage or effect haunt as above.
How can the PCs know about the weaknesses of the specific haunt?
If the haunt is just 1 round haunt, its after surpise round in resetting mode, cannot be damaged by the PCs and can be communicated with, e.g. spirit planchet, knocking or something else.
The PCs do not receive XP as haunt was not defeated.
After reset time repeat from start, HP of haunt are still the reduced value from prior combat.
If the haunt is persistant or repeating, he effects again targets on ini 10 next round. Rules state:
"On the surprise round in which a haunt manifests, positive energy applied to the haunt (via channeled energy, cure spells, and the like) can damage the haunt’s hit points (a haunt never gains a Will save to lessen the damage done by such effects, and attacks that require a successful attack roll to work must strike AC 10 in order to affect the haunt and not merely the physical structure it inhabits). Unless the haunt has an unusual weakness, no other form of attack can reduce its hit points."
That implies that after the surprise round positive energy damage cannot damage the haunt. It seems to imply that his weaknesses can still damage it. Furthermore specific other acts could damage it, if description allows (im thinking about some haunts in carrion crown).
About persistant haunts:
"Some haunts are persistent, and their immediate effects continue beyond the surprise round into actual full rounds. Persistent haunts continue to trigger their haunt effects once per round on their initiative rank until destroyed or they no longer have a target. All primary effects created by a haunt are mind-affecting fear effects, even those that actually produce physical effects. Immunity to fear grants immunity to a haunt’s direct effects, but not to secondary effects that arise as a result of the haunt’s attack."
So the effect continues until the haunt is reduced to 0 HP, but after the surprise round only attacks vs weaknesses can damage it.
Therefore a persistant haunt without weaknesses, like headless horsemanhttp://www.d20pfsrd.com/gamemastering/haunts/cr-10-12/headless-hors eman-cr-10
can only be damaged in the surprise round with positive energy. If he has HP afterwards, he continues with his effect each round forever, as no targets in area does not cause a haunt to become dormant (at least rules do not state this) and he can not be damaged in any way.
The only way to stop it, would be to fulfill its destruction condition, which could be a bit difficult with the permanently active and undamagable haunt.(You have to bury a holy sword in his area of effect.)
That somehow does not add up.
How does damaging haunts work?
Can they be damaged by positive energy in each roun, in which they act to effect something?
Do they become dormant, when no target is in the area?
Can 1 round effect haunts be damaged in their reset time?
