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 Orcs, who had been uplifted by the Gnomes 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.