Astrid III is a planet in the Koprulu sector.
It was occupied by both the Terran Dominion and a feral zerg brood during the Second Great War. At some point, the feral zerg developed the ability to morph zerglings from larvae at an extremely fast rate. Abathur discovered this evolution, and sought to assimilate it into the Swarm.
Kerrigan telepathically seized control of the local hive cluster. Abathur then incorporated the feral zerg's adaptation into the Swarm, and created the swarmling strain. Kerrigan very quickly spawned a massive army of swarmlings which she used to defeat the Dominion forces attacking the hive cluster. She then had the swarmlings attack and destroy the Dominion base nearby.