From the introduction: When the zerg first arrived in the Koprulu sector, they were unified by their absolute obedience to the zerg collective sentience known as the Overmind. The Overmind directed the actions of every zerg creature in the Swarm, functioning through a hierarchy of lesser sentients. Although the Overmind was primarily driven by its desire to consume and assimilate the advanced protoss race, it found useful but undeveloped material in humanity.