The Forsworn, also known as "Reachmen", are the original inhabitants of the northwestern region of Skyrim, which of course is eponymously known as The Reach. They are a large underground cult composed primarily of Bretons, led by Madanach, the "King in Rags". They occupy ancient structures and underground caverns throughout The Reach in northeastern Skyrim. They are the modern-day remainder of an ancient culture that existed in Skyrim before that of the Nords. They are absolutely hostile to all other groups and factions, contain only humans in their ranks though they seem to have some sort of alliance with Hagravens, and are dedicated solely to the reacquisition of Skyrim as their homeland, to the exclusion of all other cultures and peoples.
The Forsworn are descended from settlers from High Rock, explaining their great similarity to Bretons, especially the preternatural power of their female shamans. Exploiting their resemblance to Bretons, they are very often able to infiltrate modern society as agents and provocateurs. As Reachmen, most are identifiable by facial tattoos.
Reachmen, while not always members of the rebellion, count as nearly the entire membership of the Forsworn, the dissident army opposing both the Imperial Legion and the Stormcloaks in their battle for the independence of The Reach.
Their culture appears to be matriarchal in form, in keeping with many European pre-Christian paganic cultures, hence their leadership by the Hagravens and female shamans. The males serve as warriors, some in elite ranks.
The Forsworn only interact with other cultures through violence and aggression, as their aim is to drive everyone but their own people from Skyrim, therefore they do not engage in trade nor any venue of cultural interpollination whatsoever.
For a brief, two-year period while the Empire was in turmoil, recovering from the assault of the Thalmor, the people later known as the Forsworn quietly took control of The Reach and presided over a peaceful civilization that showed much promise. There was even mounting favor of applying to the Empire as a component culture. However, Ulfric Stormcloak elected to begin building his Skyrim-wide powerbase by smashing this foundling society, ruthlessly annihilating resistors to the last man, woman and child, along with any supporters amongst other inhabitant groups. This event is described in the book The Bear of Markarth.
Hagravens can apparently transform male warriors, upon their consent, into magic-infused warriors known as 'Briarhearts'. The process replaces these individuals' hearts with artifact hearts that in fact look like reddish briars; the process is also sometimes known to fail, leaving candidates dead on the hagraven's operating table.