Rachni

The rachni are an extinct insect-like species from the planet Suen that threatened Citadel space roughly two thousand years ago during the Rachni Wars. Intelligent and highly aggressive, the spacefaring rachni were driven to expand and defend their territory. They were eventually defeated and completely eradicated by the krogan, who had been uplifted by the salarians for their combat prowess and physical resilience to directly confront the rachni in the harsh environments of the rachni worlds. The accidental discovery of the rachni led to the Citadel races curbing their rapid expansion, in fear of being plunged into another galactic war.

Subtypes

 * Rachni Worker - Workers are the smallest type of rachni. They appear as small green enemies and their main attack is a suicide explosion that releases toxic damage over a wide radius.
 * Rachni Soldier - Soldiers are slower, but much larger creatures than Rachni Workers, with thin tentacles ending in little pods. Soldiers are the main defenders of rachni held areas and can cause toxic damage with their spit.
 * Rachni Brood Warrior - Brood Warriors are rachni who are much larger and stronger than normal Rachni Soldiers. They are male gendered rachni and they have some biotic ability.
 * Rachni Queen - Queens are the largest and most intelligent of the rachni. Queens lead the rachni and provide guidance for the rest of their species.

Origins
The rachni, a species of spacefaring insects guided by a hive-mind intelligence, evolved on Suen, a planet tidally-locked to its red dwarf star. Life developed in a habitable terminator zone between hemispheres that were constantly scorched or perpetually frozen. The harsh conditions of Suen's surface forced the rachni to forage underground amid sprawling subterranean river systems.

Sometime at the height of the Prothean Empire, the Protheans discovered the rachni and bred them as weapons of war, selecting for the most cunning and warlike queens and unleashing rachni swarms on their enemies. Eventually, the rachni became too difficult to control and turned on their Prothean masters. The Protheans attempted to eradicate the rachni and destroyed as many as 200 worlds before they were satisfied the rachni were extinct, but enough survived on Suen to replenish their population.

As the rachni developed civilization, they established surface cities on Suen. Wrestling with the engineering problems of Suen's extreme surface environment may have given the rachni their first insights into how to build interstellar spacecraft. After achieving spaceflight, the rachni first traveled to the neighboring planet of Kashshaptu and discovered a crater there known as "the howling gulf", which featured an abundance of element zero. The rachni obtained samples of the eezo and studiously researched its properties, but would not develop FTL technology until centuries later.

The Rachni Wars
Around 1 CE, salarian explorers opened a mass relay to a previously-unknown system and encountered the rachni, who promptly captured them. The rachni used their extensive research on element zero to reverse-engineer the FTL drives of the explorers' starships. They proceeded to construct FTL vessels of their own and rapidly expanded into the galaxy, ushering in the Rachni Wars.

Attempts by the Citadel races to negotiate were futile, 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.

Mass Effect
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 favor releasing the rachni, claiming the krogan went too far while Garrus advocates alerting the Council and is against committing wholesale genocide, and Kaidan Alenko favors 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.

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. She also suggests that something goaded the rachni into war with the galaxy against their will, since the rachni are apparently not an inherently aggressive race.

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. 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.

When the queen was discovered during the mission, 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 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 betray Shepard, resulting in the loss of the Rachni Workers War Asset and damage to several Alliance War Assets.

Even after the situation on Uttuku 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.

Leviathan
While searching for the mythical Leviathan, one of Dr. Garret Bryson's ideas to track the creature is to use galactic activity patterns of the ancient rachni at the time of the Rachni Wars. Bryson theorized that Leviathan may have been responsible for provoking the rachni in a bid to create an army to face the Reapers in the next harvest, and so rachni activity would correlate to Leviathan activity. The theory turns out to be irrelevant in the search for possible Leviathan sites, however, and whether Bryson surmised correctly that Leviathan caused the Rachni Wars is unknown.

Culture
The rachni are a territorial race, determined to remain isolated from the rest of the galaxy. They normally inhabit extremely hazardous worlds, able to survive environments that would kill most sentient species. 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 color. According to Dr. Ann Bryson, 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 color 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.

Trivia

 * The rachni resemble several insectoid alien races in various science fiction series, but most closely resemble the Arachnids from Robert A. Heinlein's Starship Troopers. Similarly, the rachni workers somewhat resemble the beetle shaped diggers from the Starship Troopers movie. The name 'rachni' even forms part of the name 'Arachnids', the scientist Flores may also be a reference to Private Dizzy Flores. The story of nearly complete extinction of a telepathic, sentient insectoid race and then a single Queen egg surviving to resurrect the race may be inspired by the Formics in Orson Scott Card's Ender's Game and subsequent sequel novels.
 * The rachni can be heard 'singing' near the wreckage where the Asari Writings are found on Altahe, at the abandoned mining camp on Nepmos, at the abandoned camp on Nodacrux, near the landing zone on Rayingri, near the downed recon probe on Eletania, on the hill near the crashed CCCP Luna 23 on Luna (where it should be impossible for any rachni to be present), and on the hill to the west of the thresher maw and crashed probe on Xawin. It is possible that BioWare used the same audio-library that Epic Games used to create Unreal 2: The Awakening, because there the same sound can be heard perfectly on the planet Severnaya.