Calus was a planet occupied by both the Terran Dominion and the Zerg Swarm during the Second Great War. The Dominion stationed a small security detachment here, situated high in the cliffs where they were impervious to attacks by zerg ground forces.
Abathur discovered that the native karak on the planet had the ability to leap incredible distances - a trait he wished to integrate into the zergling strain. After assimilating the karak's essence into the Swarm, Abathur created the raptor zergling strain, capable of reaching the Dominion outpost. On Kerrigan's orders, the new raptor zerglings attacked the outpost and destroyed it.