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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. By taking a powerful terran psionic, Sarah Kerrigan, the Overmind evolved a new and unique creature: the Queen of Blades. After the protoss destroyed the Overmind during the invasion of Aiur, the Queen of Blades eventually came to dominate the Swarm.

The zerg are entirely unlike the terrans and the protoss. They are composed of many different species that were integrated into the Swarm via zerg infestation. Such creatures are rapidly and selectively evolved into ruthless killing machines to further the driving zerg imperative of achieving absolute domination. The zerg make no use of technology to create their weapons, armor, or starships. Instead, these functions are efficiently fulfilled through biological adaptation and planned mutation of zerg strains. Even zerg structures are in fact specialized organs within the living, growing organism of a zerg nest.