Rachni

The rachni are an extinct insect-like species that threatened to defeat and overthrow the Citadel Council roughly two thousand years ago. They were defeated and completely eradicated by the krogan, who had been groomed by the salarians to effectively do the Council's dirty work. This indirectly caused the Krogan Rebellions much later on.

Though now extinct, the rachni once threatened every species in Citadel space. Explorers foolishly opened a mass relay to a previously-unknown system and encountered something never seen before or since: a species of space-faring insects guided by a hive-mind intelligence.

Unfortunately, the rachni were not peaceful, and the galaxy was plunged into a series of conflicts known as the Rachni Wars. Attempts to negotiate were futile, as it was impossible to make contact with the hive queens that guided the race from beneath the surface of their toxic homeworld.

The emergence of the krogan ended the Rachni Wars. Bred to survive the harshest environments, the krogan were able to strike at the queens in their lairs and reclaim conquered Council worlds. But when krogan fleets pressed them back to their homeworld, the rachni refused to surrender, and the krogan eradicated them from the galaxy.

The rachni culture appears to be similar to that of ants or bees with the addition of biotic abilities. The rachni themselves come in four varieties. Rachni workers are small, fast and explosive. Rachni soldiers are slower, but much larger, creatures with thin tentacles ending in little pods. These tentacles are used to impale enemies - the rachni can use weapons but prefer to use these tentacles. There is a larger, more powerful version of the rachni soldier called a rachni brood warrior, the senior males of the hive (only encountered in a side assignment).

Finally, there are rachni queens. Queens are said to be able to lay an egg every few hours which can result in an entire rachni nest in days. The reproductive processes of the rachni do not require a male counterpart because the queens carry the genetic information of their fathers inside them, allowing them to lay eggs even if no brood warriors are present. (Rachni eggs are about the size of a football and can be destroyed with one shot.) Queens resemble soldiers but are much larger, with darker skin and glowing eyes. They carry the genetic memory of their mothers and can control the minds of receptive beings like asari commandos.

The rachni aren't as extinct as we've 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 warship 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 she had reached maturity, this resurrected queen began laying eggs, but the rachni were removed from her presence by Binary Helix once hatched, in an attempt to control them. As the queen later explains to Shepard, the rachni elders communicate by 'singing' and this comforts their children. Without the queen's 'song' to soothe them, the rachni soldiers and workers have been driven insane from fear and attack anything on sight. They are beyond help, so the queen 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". However, she mentions something about 'a tone from space' that forced the rachni to "resonate with its own sour yellow note", hinting that there may have been more to the Rachni Wars than the Council were aware of (perhaps even that the rachni were affected by Sovereign's indoctrination). 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. If they are present, both Wrex and Ashley are angry at the prospect of releasing the queen - 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 have rachni imprisoned on Binthu as part of their 'super soldier' research. More rachni are encountered on Nepmos, at Listening Posts Alpha and Theta. They have infested various sites across Nepmos and have taken over a nearby mine. After clearing them 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 a scientist 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.

Tactics
Rachni workers and soldiers use bio-attacks, which appear to be acid-based: workers rush up to the squad and explode, while soldiers spit acid from a distance. These attacks ignore kinetic shielding and Barrier, and directly damage health. The best tactic with workers is to take them out from a distance - a single shot will make them explode - and Throw them back if they get too close. Soldiers are tough and deadly but vulnerable to biotics. Lift will prevent them using their acid attack and allow them to be hit with Warp, Neural Shock or a hail of assault rifle fire.