Suen, the rachni homeworld, is tidally locked to the system's red dwarf. With one side constantly bombarded by the sun and the other completely frozen, life evolved in the terminator, the perpetual twilight zone between Suen's two extremes. The rachni mainly foraged underground: Suen's abundant subterranean rivers presented them with a place to thrive. 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.
Suen was the site of the last battle in the Rachni Wars. When krogan forces bombarded the planet, the rachni abandoned their surface buildings for the security of their tunnels. The krogan, resistant to Suen's toxic atmosphere, carried weapons of mass destruction into the secret chambers of the rachni queens, and detonated explosions so powerful the sinkholes are still visible today.