Since Maia stated that she'd recount everything she had learned, I'll just put those two spoilers in this post for convenience:
With a combination of skill and a bit of luck, you quickly come upon a number of records on the topic of Harrowstone. From the various records you find, you're able to piece together quite a bit of information about the place.
Built in 4594, Harrowstone was one of Ustalav's most infamous prisons. It was constructed to house the worst of the worst criminals from the many smaller jails about the country, often to await death by hanging, but always to live out the rest of their lives as prisoners. In fact, the town of Ravengro itself was founded because of the prison, as a place where guards and their families could live, and to produce food and other supplies for the prison.
Harrowstone has now stood ruined for over 50 years, following a fire in 4661. The fire killed all of the prisoners and most of the guards, as well as the warden and his wife (although none of the records provide a reason for why she was there at the time). A large portion of the prison's underground eastern wing was destroyed in the fire, but most of the stone structure aboveground was left relatively intact. A statue commemorating the warden and the guards who lost their lives was built in the months following the tragedy, and placed by the river.
At the time Harrowstone burned, five particularly notorious criminals had recently arrived at the prison, and the fire itself came about during a prison riot presumably staged by one or more of these five only a few months after their arrival. One record states that "while the commonly held belief - in fact, the story told to the public - is that the tragic fire began accidentally during the riot, in truth the prisoners had already seized control of the dungeon and had been in command of the lower level for several hours before the fire. Warden Hawkran triggered a deadfall to seal the prisoners below, but in so doing trapped himself and nearly two dozen guards. The prisoners were in the process of escaping when the few panicked guards remaining above started the fire in a desperate attempt to end the riot."
With some digging, you're able to uncover the monikers of all five of the prisoners in question; Father Charlatan, The Lopper, The Mosswater Marauder, The Piper of Illmarsh, and The Splatter Man.
You're also able to uncover more detailed knowledge regarding two of the five, but similar information about the remaining three seems to be harder to locate, and would likely require at least another day of study to find.
- Father Charlatan (real name Sefick Corvin) was the only one of the five who wasn't technically a murderer, yet his crimes were so blasphemous that several churches demanded he be punished to the full extent of Ustalavic law. Although he claimed to be an ordained priest of any number of faiths, Father Corvin was in fact a traveling con artist who used faith as a mask and a means to bilk the faithful out of money in payment for false miracles or cures. He became known as Father Charlatan after his scheme was exposed and his Sczarni accomplices murdered a half-dozen city guards in an attempt to make good the group's escape.
- The Mosswater Marauder (real name Ispin Onyxcudgel) was originally a well-liked artisan and a doting husband. When he discovered his wife's infidelity, he flew into a jealous rage and struck her dead with his hammer, shattering her skull and his sanity with one murderous blow. Wracked with shame and guilt, Ispin became convinced that if he could rebuild his wife's skull, she would come back to life - but unfortunately, he could not find the last blade-shaped fragment from the murder site. So instead, Ispin became the Mosswater Marauder. Over the course of several weeks, the cunning dwarf stalked and murdered nearly 20 people while searching for just the right skull fragment. He was captured just before murdering the daughter of a visiting nobleman from Varno, and was carted off to Harrowstone that same night.
And to directly ward off a possible misunderstanding: There were a lot more than five prisoners within at the time.