Rachni

The rachni were an intelligent, insect-like species native to the planet Suen, guided by a hive-mind intelligence centered on their queens. Roughly two thousand years before the era of Commander Shepard, their rapid expansion plunged Citadel space into the Rachni Wars, a conflict the Council races could not win until they uplifted the krogan to fight in the rachni's brutal homeworld environments. Declared extinct in 300 CE, the rachni proved far from gone, and a lone surviving queen would later force a choice over whether the species deserved a second chance.

Biology and Subtypes#

The rachni had chitinous exoskeletons, four limbs for walking, and bled a green fluid, though the number of eyes, arms, and tentacle-claws varied between their specialized forms. Most forms bore a beak of five mandible-like protrusions in place of a mouth. The species was divided into distinct castes. Workers were the smallest. Soldiers were larger and slower, serving as the main defenders of rachni territory and capable of spitting toxic fluid. Brood warriors were larger and stronger still, male-gendered rachni with some biotic ability. The queens were the largest and most intelligent of all, leading the hive and providing guidance for the rest of the species.

Origins on Suen#

The rachni evolved on Suen, a world tidally locked to its red dwarf star, where life developed in a narrow habitable terminator zone between hemispheres that were perpetually scorched or frozen. The hostile surface drove the rachni to forage underground amid sprawling subterranean rivers. At the height of the Prothean Empire, the Protheans discovered the rachni and bred them as living weapons, selecting for the most cunning and warlike queens. The rachni eventually became impossible to control and turned on their masters, prompting the Protheans to destroy as many as two hundred worlds in an attempt to wipe them out. Enough survived on Suen to replenish the population. As the species rebuilt, it raised surface cities, achieved spaceflight, and discovered abundant element zero in a crater on the neighboring planet of Kashshaptu, though faster-than-light travel would not come for centuries.

The Rachni Wars#

Around 1 CE, salarian explorers opened a mass relay into a previously unknown system and were captured by the rachni, who used their existing element zero research to reverse-engineer the explorers' faster-than-light drives. The rachni built their own fleets and expanded rapidly, igniting the Rachni Wars. Diplomacy proved futile, as the queens who guided the species could not be reached beneath the inhospitable surfaces of their worlds, and the Council races assumed the rachni were irredeemably hostile. The rachni held the advantage, overwhelming defenses through sheer numbers, until the krogan emerged in 80 CE. Able to survive the harshest environments, the krogan struck directly at the queens in their lairs and reclaimed conquered worlds. When the rachni refused to surrender at Suen, the krogan bombarded their cities and detonated bombs in their underground nests, opening massive sinkholes across the planet. The rachni were declared extinct in 300 CE, though the Council left a listening post in the system as a precaution.

Resurgence at Peak 15#

The rachni proved less extinct than the galaxy believed. At the remote Peak 15 facility on Noveria, Shepard encountered rachni workers and soldiers that had escaped from the laboratories of Binary Helix, a company that had recovered a rachni egg in cryogenic suspension from a derelict ship. The egg hatched into a queen, and Matriarch Benezia used its genetic memory to locate the Mu Relay on the orders of Saren Arterius. The captive queen explained to Shepard that rachni elders soothe their young by singing, and that without that song the workers and soldiers had been driven mad by silence. She asked Shepard to euthanize them, then offered the Commander a choice over her own fate: kill her with the laboratory's acid tanks, or release her. She claimed innocence of the old war, having been only an egg when it was fought, and promised that if freed she would seek a place to live in peace and teach her children forgiveness. Companions argued both for and against mercy, but the final decision rested with Shepard.

The Reaper War and Final Fate#

The rachni's fate carried forward into the Reaper invasion. A rachni hive was discovered on Utukku, where it emerged that the Reapers had found the queen and indoctrinated most of her children, converting them into walking artillery called Ravagers that spawned mutated workers known as Swarmers. If the queen had been spared on Noveria, she herself resisted indoctrination, and Shepard could again choose to save or abandon her at Utukku. A saved queen sent her workers to aid the Crucible project, prized for their engineering skill, while an abandoned queen accepted her end and embraced the silence. If the original queen had been killed, the Reapers instead constructed an artificial queen thrall, which Shepard could spare only to see it eventually betray the Alliance. Even after the Utukku situation was resolved, Ravagers created earlier continued to appear on battlefields across the galaxy.

Culture and Communication#

The rachni were a fiercely territorial and isolationist species, inhabiting hazardous worlds and responding to any intrusion with swift, brutal force. Many dismissed them as mindless animals, but they were highly intelligent, having achieved spaceflight, cryogenic suspension, and the conquest of vast territory while carefully nurturing their soldiers rather than treating them as disposable. Their queens perceived the world through a synesthetic blend of sound and color, and their communication was theorized to rely on pheromones and an organic quantum entanglement communicator rather than telepathy. A queen could biologically link with nearby living or recently deceased organics to produce speech, an act she likened to plucking the strings of an instrument. The rachni regarded thought and speech as forms of music, speaking of death as "the great silence" and calling Suen the "singing planet." Their ability to share knowledge genetically across generations made each queen a vast repository of their species' memory.