The Sodan were a sect of Jaffa who rebelled against their 'god' over five-thousand years ago.
For the longest time, the Sodan were part of an elite force under the command of the Goa'uld known as Ishkur. For years, they pillaged and plundered under his name, driving fear into the hearts of all those that would oppose his rule. The further they rose among the Jaffa ranks, the more they were able to discern the truth: that he was not a god at all, but merely a being of flesh and blood masquerading as one. Ishkur branded the now rebelling Sodan as traitors, and ordered their deaths. They tried to fight, to show their brothers the faith, but the odds were too great and they were forced to flee. They set out to find Kheb, hoping that their souls would find eternal peace. Instead, their search led them to P9G-844, where they took up residence until they were discovered by the Tau'ri. (SG1: "Babylon")
In order to acquire symbiotes that could sustain them, they raided Goa'uld strongholds for shipments of symbiotes that had yet to be implanted in any Jaffa, and such missions became rites of passage for their youngest warriors. By the time of their contact with SG-1, the Sodan's numbers were such that two hundred warriors was a sizable force for them, thus suggesting that they lived in a number of scattered villages. (SG1: "Babylon")
Their beliefs would eventually lead them to believe that the true gods were those that came long before the Goa'uld: the Ancients. Many of the Sodan, including the leader Haikon, believed that only the Ori could provide them with the necessary evidence of their gods' powers, even when they came to realize that the Ori themselves were not the Ancients. (SG1: "Babylon", "The Fourth Horseman, Part 2")
Eventually, however, the Sodan came to understand that the Ori were in fact no better than the Goa'uld. They renounced Origin, and assisted the Tau'ri in subduing a Prior to prevent the Ori plague from destroying all life on Earth.(SG1: "The Fourth Horseman, Part 2")
In retaliation for their abandonment of Origin, a Prior infected the Sodan warrior Volnek with a different illness. After going insane - and then apparently dying - Volnek systematically wiped out his entire village, before Teal'c and Lieutenant Colonel Cameron Mitchell literally blew Volnek apart. The only Sodan to survive the rampage (barely) was Lord Haikon himself. (SG1: "Arthur's Mantle")