In the first Mass Effect game, the geth forces which are being led by Saren Arterius are not a part of the primary geth collective. They are a splinter faction which have been dubbed as "heretics" by the main geth consensus for choosing to follow the Reaper known as Sovereign in its mission to usher in the next Reaper invasion of the Milky Way.
History[]
300 years after the Morning War, the geth were approached by Nazara (the Khelish word for "sovereign" or "supreme ruler"), the vanguard of the Old Machines (the geth's name for the Reapers). Nazara offered the Geth Reaper technology that would allow the Geth to achieve their ultimate goal of unification, the development of a megastructure that would house the entire geth AI network. In exchange, Nazara required the assistance of the geth in capturing the Citadel space station and ushering in the next Reaper invasion. The majority of the geth rejected the offer, preferring to achieve their goal via their own means instead of relying on others. However, some geth believed that Nazara was a bringer of salvation and accepted its offer, splintering from the primary geth network and agreeing to assist the Old Machine and its Turian agent Saren. These geth separatists would then become known to the mainstream geth as "heretics".
The heretics came to regard Nazara as some sort of machine god, the pinnacle of synthetic evolution. Nazara itself, however, was disgusted by the geth's worship of it, though it saw their value as tools to further its goals. They aided Nazara and Saren in many engagements, such as the attack on Eden Prime, and the Battle of the Citadel. After Nazara was destroyed in the latter battle, the heretics lost much of their menace, and their operations outside of the Perseus Veil were quickly mopped up by Council forces. The heretics did maintain operation of a large space station within the Terminus Systems, located in deep space between stars.
The primary geth consensus was not content with simply ignoring the activities of the heretics and considered them a potential threat. Through monitoring the extranet transmissions in Council space, they became aware of Commander Shepard, an officer of the Human Systems Alliance who was responsible for the defeat of Sovereign at the Citadel. Realizing his/her value as a possible ally, the geth deployed a specialized platform that housed 1183 Geth programs and sent it on a mission to track down Shepard.
((The following events only occur if the player recruits Legion in Mass Effect 2.))
The Legion platform encountered Shepard aboard the carcass of a dead Reaper and was brought aboard the Normandy SR-2. Shepard recruited Legion and from it learned about the social state of the geth and how they had become divided. Legion informed the commander of the heretics' main base of operations and requested assistance in dealing with the heretic threat.
Depending on the decisions of the player during Legion's loyalty mission, the heretics will either be reprogrammed and rejoin the mainstream geth collective, or they will be wiped out altogether when Shepard destroys the Heretic Station.