The Mecrosa Order Immediately following their conquest of the Sith people (circa 7,000 years before the Battle of Yavin), a few upstart Exiles, now self-proclaimed Sith Lords, returned to Republic space believing they could revenge themselves on their Jedi enemies with their new knowledge of Sith magic. The attack proved premature, however, and instead they succeeded only in revealing that some Jedi schismatics had found a new home and a people to rule outside of charted Republic space. The true scale of the Sith menace lurking in unknown space remained invisible for millennia, however. Two thousand years later, the Great Hyperspace War indelibly proved to the Jedi how dangerous the Sith Lords could be.
The thoroughness of the Jedi extermination efforts following this first full-scale Sith war was criticized by many as being executed with a gusto rivaling a Bothan declaration of Total War. Nevertheless, Sith teachings survived, and a thousand years later, a new war erupted, this time called the Great Sith War. This conflict came to be grouped with a series of major Sith-related encounters that followed soon after: the Mandalorian Wars, the Jedi Civil War, and the Cleansing of the Nine Houses.
The Mecrosa Order was the focus of the Cleansing. The order started out benevolently enough. Emerging during the Tapani sector's Dynastic Era, approximately a quarter millennium before the Great Sith War, the Mecrosa began as an order of nobles from House Mecetti who took an oath to protect the house against external attack and insurrection. For the next two hundred years, Mecrosa flourished from its castle on the planet Nyssa, gaining wealth and erecting chapter houses and fortresses on Mecetti worlds. To assist in its crusade, the order also oversaw the creation of an elaborate spy network. It is believed that the virtues of the Mecrosa Order were compromised somewhere during this two century stretch. Jedi from rival House Pelagia insisted that the order's founder, Viscountess Mireya, was the root of Mecrosa's Sith corruption.
When Mecetti High Lord Tritum XI realized there was a threat of inbreeding facing his house, he sought to introduce new noble blood. In one of those all-too-common circumstances when politics and noble pride must be reconciled, Tritum looked to a suitor outside the Tapani region -- to the nobility of a world called Vjun. Vjunite Viscountess Mireya brought to House Mecetti not only her noble pedigree, but Sith teachings as well. Sith rituals were subtly integrated into Mecrosa initiation rites and ceremonies, and soon Sith "lords" of a very different breed terrorized Tapani sector, assassinating Tapani house leaders that appeared to threaten Mecetti power and infecting Pelagia's Jedi with Sith poisons.
(source: Evil Never Dies: The Sith Dynasties)