Re: OOC
Everyone in the party has at least some ranks in Knowledge: Religion, and you've been doing so much research on ghosts in general and the prisoners specifically that you won't have to make a check.
Ghosts can be created by necromantic magic like create greater undead, but more commonly they are created when someone dies in exceptional circumstances. A violent death, a creature killed while driven by unfinished business, someone who dies in the throes of intense sadness or longing or anger can all cause the deceased to linger on as a ghost.
Like most undead almost all ghosts have a violent hatred for the living, and you know that in the case of Harrowstone the ghosts were murderers and evil men before they perished.
Ghosts can have a variety of unique and terrifying powers, usually tied to their death or to the abilities that they had in life. So far it seems like the ghosts can reach out to affect the town. You have personally faced some of these haunts, like the possessed Gibs.
Getting rid of ghosts can be difficult. Simply defeating a ghost in combat will usually only disperse their energies, they will typically re-form within 2d4 days. Using something tied to them or their deaths to defeat them is a permanent solution, as is discovering and resolving the unfinished business that keeps them on the mortal plane.
So you know the manner in which they possess their victims (ghost can just do stuff like that), why they were chosen and how they cling to the mortal plane (the Harrowstone fire was a traumatic event likely to create ghosts in any case, and these five were the most powerful and malevolent souls that perished).
You also know why they would seek vengeance, undead hate the living in general and these ghosts were murderers and maniacs.
The most odd question is: If the Harrowstone fire was fifty years ago, why have the ghosts begun affecting the town so strongly only now? You know that any ghost killed in the fire would have manifested within a couple of days of the event.
You know that the prison has a reputation for being haunted, people reporting hearing a woman sobbing in the walls at night, but were no hauntings and supernatural incidents until the night the Professor died.