The Rachni were known to have existed during the time of the Prothean Empire, discovered living within the terminator zone of the planet Suen . Ever seeking ways of expanding their dominion, the Protheans selected the most vicious Rachni Queens to rear and breed, creating armies of useful biological weapons to unleash upon their enemies. The Protheans' abuse of the Rachni would not last, however, as the creatures eventually became too difficult to control and turned on their masters. Following this betrayal, the Protheans took drastic measures to destroy the Rachni and razed over 200 planets before they were satisfied that the entire race had been exterminated. However, enough Rachni survived in the deepest subterranean levels of Suen for them to repopulate and continue to develop into a civilization. By the time they developed space travel, their former oppressors the Protheans had been wiped out when the Reapers had carried out their harvest of all the advanced organic races in the galaxy of that cycle.
The Rachni Wars
In the year 1 CE, the Rachni were discovered by the Salarians. The Salarian explorers were captured and the Rachni, having previously studied element zero on their neighbouring planet Kashshaptu, conducted extensive research on the Salarians' technology and reverse-engineered it to produce faster-than-light vessels of their own, allowing the Rachni to rapidly expand across the galaxy. This was the beginning of the Rachni Wars .
The races of the Citadel Council attempted to communicate with the Rachni without success, as it was impossible to make contact with the hive queens that guided the race from beneath the inhospitable surface of their homeworld. It was assumed that the Rachni were irredeemably hostile and could only be stopped through warfare, but the Rachni had the upper hand and overwhelmed defenses with their sheer numbers.
The emergence of the Krogan in 80 CE turned the war against the Rachni. Able to survive the harshest environments, the Krogan were able to strike at the queens in their lairs and reclaim conquered Council worlds. When Krogan fleets pushed them back to Suen, the Rachni refused to surrender. The Krogan response was swift and brutal: they bombarded Suen's cities and detonated powerful bombs in the Rachni's underground nests, creating massive sinkholes on the planet's surface. The Rachni were declared extinct in 300 CE, although in caution the Citadel Council left a listening post in the system to monitor for any survivors.
In the year 2381, it is revealed that the Rachni aren't as extinct as the galaxy had been led to believe. At the remote Peak 15 facility on Noveria, Commander Shepard encounters Rachni workers and soldiers that have escaped from the labs. The company renting Peak 15, Binary Helix, had found a Rachni ship adrift in space carrying eggs in cryogenic suspension. One of the eggs was recovered and brought to Noveria for study. Binary Helix planned to clone the Rachni inside and create an army, but when the egg was hatched the researchers realized this was no ordinary egg, but the egg of a Rachni queen. Matriarch Benezia used the genetic memory of the queen to find the location of the Mu Relay on the orders of Saren Arterius.
After the queen had reached maturity, she began laying eggs but, the newborn Rachni were removed from her presence by Binary Helix once hatched, in an attempt to control them. This same queen later explains to Shepard that Rachni elders speak to their children by 'singing', this song soothes and comforts them. It is apparent that without a song from their elders, Rachni workers and soldiers are driven insane by the silence and will attack anything on sight out of fear. The queen believes they are beyond help and asks Shepard to euthanise them.
The queen then asks what Shepard intends to do with her: either use the acid tanks in the lab to kill her, or let her go. She says she knows nothing about what happened in the war: "we were only an egg, hearing mother cry in our dreams". The queen promises that if she is released, she will not attack other races again but find somewhere to live in peace, and teach her children about Shepard's forgiveness. Liara and Tali'Zorah nar Rayya favour releasing the Rachni, claiming the Krogan went too far while Garrus advocates alerting the Council and is against committing wholesale genocide, and Kaidan Alenko favours releasing the Rachni only because he thinks killing a whole race is counterproductive to humans staying out of old galactic grudges. However, both Wrex and Ashley are angry at the prospect of releasing the queen, if they are present alongside a more compassionate squadmate - Wrex isn't impressed by "bugs writing songs about you" and Ashley wants to protect her sisters from horrors like the Rachni, not release more of them - but the final decision is up to Shepard.
The Human supremacist group Cerberus created cloned Rachni on Binthu as part of their super-soldier research. More Rachni are encountered at Alliance listening posts on Nepmos and Altahe. After clearing the Rachni out, Shepard discovers they were specimens shipped from Noveria by Cerberus, and tracks them back to a depot in the Gorgon system. The depot is overrun with Rachni and there are no survivors. After killing the Rachni, Shepard finds the logs of the station's commanding officer named Flores who was involved in the project. She angrily claims they treated the Rachni like animals when they should have treated them like POWs and leaves a recommendation that Cerberus pursue other projects; the Rachni are too smart.
Mass Effect 2
If the Rachni Queen was spared, Shepard will encounter an Asari on Illium who is in contact with the Rachni. The Asari tells Shepard that the Queen believes Shepard is fighting the enemy who "soured the song of" the Rachni. She also states that the queen is building an army to fight them when they arrive in force, based on an otherwise unexplored planet. Shepard concludes that the enemy the message refers to is the Reapers. Additionally, a news report reveals several mysterious ships have been sighted that match Rachni designs; the Citadel Council has pledged to investigate these sightings. This appears to indicate that the Rachni have progressed extensively in the two years since Shepard freed the Queen. The Asari on Illium confirms that they have at least advanced sufficiently to be capable of repairing a damaged starship.
If the Rachni Queen was destroyed, the Rachni are effectively extinct again. A news report explains that a Rachni cloning facility was discovered on Noveria, but no samples of the Rachni have been found and the attempt to clone the Rachni appears to have been unsuccessful.
Mass Effect 3
During the Reaper invasion , a Rachni hive is encountered on Utukku by the Krogan Aralakh Company, led by Grunt, if he survived the Suicide Mission in ME2, or Urdnot Dagg. It is revealed that the Reapers found the Rachni Queen and indoctrinated most of her children, though not the queen herself. The indoctrinated Rachni were converted into Ravagers , walking organic artillery, through a process of implantation and genetic modification. Ravagers also bear egg sacs that spawn mutated Rachni Workers known as Swarmers.
The fate of the Rachni is once again in Shepard's hands if the Commander chooses to deal with the Utukku hive. Shepard has a choice to either save the queen or let her die. If she is saved, Rachni workers are sent to work on the Crucible project, where they are prized assets for their engineering capabilities, industrious nature, and good teamwork. If she is left to die, the queen accepts her fate, claiming she will 'embrace the silence' and the Rachni are once more extinct.
If Shepard did not spare the Rachni queen on Noveria, the Reapers will artificially construct a Queen thrall in order to control a Rachni army. Shepard can choose to spare this artificial queen even if the Commander killed the real queen. This queen will eventually turn against Shepard, unleashing her workers against the Alliance's engineers working on the Crucible. This betrayal is dealt with swiftly: all the Rachni are killed but at cost to Alliance manpower.
Even after the situation on Utukku is resolved and the Reapers can no longer use the Rachni Queen to generate more Rachni for conversion, the numerous Ravagers created beforehand continue to appear on battlefields across the galaxy.
If Shepard chooses to activate the Crucible, the real Queen was spared twice, and the Krogan were denied the real cure for their genophage, the Rachni will be seen settling their spindly limbs on the ruins of a desolate Tuchanka.
The Rachni are highly territorial and tend to isolate themselves from other races. Their biology is extremely resilient and they are capable of surviving in almost any environment. As such, they choose to populate planets with harsh environments where most other races cannot survive. Should their territory be invaded on purpose or even by accident, they respond with swift and brutal force.
However, many make the mistake of underestimating the Rachni as mindless animals when in fact the Rachni are an extremely intelligent sentient species. The Rachni achieved space flight and a form of cryogenic suspension, developed weapons and carved out a huge swathe of galactic territory. Far from being treated as disposable resources of the hive, Rachni soldiers are carefully nurtured as part of the group.
The queens are the leaders of the Rachni hive mind. Their ways of perception, thinking and communications seem to be different from that of other races, and based upon a synesthetic perception of sound as both sound and colour. Rachni communication is geared towards pheromones, and, as further proof of their intelligence, an organic quantum entanglement communicator - not, as so many believed, telepathy.
Rachni regard speech and thought as forms of music, as shown by the strange metaphors used when attempting to communicate, for example, "songs the colour of oily shadow". The speech patterns used by them in such occasions are also unusual; "When we speak, one moves all". They refer to their manner of communication as "singing", which soothes and nurtures their young, and this appears to colour their entire psychology - for example, the Rachni speak of death as "the great silence", and refer to their homeworld as the "singing planet". The Rachni ability to genetically share knowledge across generations makes queens a vast repository of information.