The sources came from a combination of books, mainly:
Pocket Guide to the Empire, 1st Edition/High Rock, which recounts that the 'Bretons' (Reachmen, Forsworn, and what other names they have been called over the years) had been the first to inhabit the area before being ousted by the Nords when the 1st Empire came about.
The Bear of Markarth, "the Forsworn Kingdom was quite peaceful for those 2 years they were in power.
True, some crimes were committed against former Nord landowners (often those accused of being the harshest towards their native workers), but on the whole the Forsworn ruled their lands fairly, and were making overtures to be recognized by the Empire as a legitimate kingdom."
With minor references from The Great War confirming time periods.
If you want to come up with your own paragraph and recounting of the event please feel free to do so, I'd be happy to alter or replace parts or all of it if you found better material.
The dwemer were the original inhabitants of the reach, not the forsworn. And the Nedes and Aldmer after them.(Which then intermingled with the elves and became bretons) The bretons would be highly offended to be lumped in with the reachmen which were said to be an intermingling of nearly every race.(including Nord)
And yet http://www.imperial-library.info/content/madmen-reach doesn't paint a pretty picture of their methods
filled with the animal skulls, severed heads, and still beating hearts that I had read about from the military reports back in the Imperial City. There, I met Cortoran, a Forsworn, who seemed amused at the prospect of me writing down his story. Which I quote in full below:
"You want to know who the Forsworn are? We are the people who must pillage our own land. Burn our own ground. We are the scourge of the Nords. The axe that falls in the dark. The scream before the gods claim your soul. We are the true sons and daughters of the Reach. The spirits and hags have lived here from the beginning, and they are on our side. Go back. Go back and tell your Empire that we will have our own kingdom again. And on that day, we will be the ones burying your dead in a land that is no longer yours."
http://www.imperial-library.info/content/legend-red-eagle
Thus was brokered to the witch: his heart, his will, his humanity. From that day forth, his was a spirit of vengeance, pitiless and beyond remorse. The rebels grew in strength and numbers, and none could stand against them. Faolan's eyes burned coldly in those days, black opals reflecting a mind not entirely his own. Two years passed, and the foreigners were all but driven from the Reach.
Such peace could not last, however, and a great host fell upon them, a swift army of invaders unlike any before. For a fortnight, Hestra's generals laid siege to Red Eagle's stronghold, till he himself came forth for battle, alone and robed in nothing but his righteous fury. A thousand foreigners fell before his flaming sword, and the enemy was routed. Yet, when night fell, so too did he. The warriors who came to him said Faolan's eyes were clear again on that final night.
http://www.imperial-library.info/content/holds-skyrim
Be aware that this dangerous region of Skyrim is home to the Forsworn, the rebellious natives of the Reach. They know the terrain, can strike without warning, and count the Empire as an enemy. If they attack, you must neither give nor expect any mercy.
But it's not just that. Parts of Bear of Markarth have been shown to be factually untrue. It throws the entire account into question.
it was no surprise that he would demand to be allowed to worship
Discredited by Igmund(Pro-Empire)
"You are with us, or you are against Skyrim" was the message on Ulfric's lips as he ordered the deaths of shopkeepers, farmers, the elderly, and any child old enough to lift a sword that had failed in the call to fight with him.
Discredited by Thonar(Pro-Thonar) and Braig(Pro-Forsworn).
Arrianus either didn't bother to do any research this time, or decided intellectual integrity wasn't important.(This isn't an argument about whether Ulfric was present at the executions. It's throwing into question the validity of the book)