Roughly two thousand years before the events in the first Mass Effect game, explorers hastily opening new Mass Relays came into contact with a race of space faring arthropods. The Rachni had been agressively expanding their empire, and after coming into contact with the council races, quickly went to war against them.
Governed by a hive intelligence, it was difficult for the council races to make any attempt to negotiate with the Rachni, as the worlds their queens inhabited were too toxic for Salarians, Asari or Turians to survive on.
Things finally shifted in favor of the Council races with the Salarians discovering the Krogan. In exchange for technology and the rights to colonize new worlds, the Salarians groomed the Krogan to battle the Rachni. With a highly versitile physiology, the Krogan were able to hunt the Rachni back to the surface of their worlds, where they could strike at the queens of Rachni colonies. Eventually, the Krogan hunted the Rachni to extinction.
Rachni are territorial creatures, and prefer to remain isolated from other species. Choosing to inhabit worlds that are too hazerdous for most sentient creatures to inhabit, when their territory is threatened, the Rachni have been known to act out very aggressively.
There is a common misconception that the Rachni are mindless beasts. They are a very intelligent species though, as they had developed technology for space travel, and had controlled a large section of the galaxy. While governed by a hive mind, the average Rachni are not mindless drones, but rather are self functioning creatures carefully nurtured by their collective intelligence.
Communication amongst the Rachni is very different from most species, as demonstrated by the metaphors they use when describing the way they speak to one another. They refer to their speech as "singing," describing it to "paint the air" around them. This form of communication seems less a direct one on one method of communication, as they describe it as "when we speak, one moves all." This communication seems critical to them, as Rachni separated from the queen and her "songs" go mad and are uncontrollably violent.
Mass Effect 1
When Commander Shepard
arrives at Peak 15 in Noveria
, he discovers Rachni have overrun the facility. While the species were believed to be extinct, it is revealed that scientists working for Binary Helix had found a derelict ship with a Rachni egg
cryogenically frozen. The original plan for Binary Helix was to clone the Rachni to create expendable soldiers, but they changed that plan when they discovered the egg contained a queen. Instead, they raised the queen to maturity in captivity, and when she began laying eggs, the scientists separated them from her, attempting to control the Rachni workers that the queen had given birth to. This resulted in the workers going insane, cut off from their queen for so long that the damage to their minds was irreversible. When asked about this, the queen finds the fate lamentable, but asks Shepard to put down these mindless Rachni. Some of these unfortunate Rachni breed on Noveria were also shipped to Cerberus
, so that they could preform tests of their own on the species.
Shepard's original reason for investigating the facility, was to track down Matriarch Benezia
, a servant of Saren
. Benezia says that the Rachni are known to store genetic memories through generations, and that the queen on Noveria had information critical for Saren's plans.
When Benezia has been defeated, the queen reaches out to Shepard, telling him of the Rachni's history, and asking him what he will do with her. Shepard is presented with the choice to kill the queen, effectively making the Rachni extinct once again, or he could release her, so that their species can start again and atone for their past actions.
Mass Effect 2
Should Shepard choose to let the Rachni queen live in the first game, there are signs that their species are beginning to recover. Various news broadcasts will mention ships that match ancient Rachni vessels being spotted, before quickly fleeing and evading further pursuit, and the Council promises to mount an investigation into the matter.
Shepard will also come across an Asari that acts as an agent for the Rachni queen, telling Shepard that they are rebuilding and will stand with him against the Reapers
when the time comes. The Asari will also mention that the old Rachni war was a mistake, and that their species is not naturally aggressive. She says that something was "souring their songs," compelling them to violence against others. It is speculated that the Reapers were manipulating the Rachni, forcing them to start a war with the other races of the galaxy.