Though now extinct, the rachni once threatened every species in Citadel space. Over 2,000 years ago, explorers foolishly opened a mass relay to a previously-unknown system and encountered something never seen before or since: a species of spacefaring 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.