Zerg

The xel'naga, an ancient and enigmatic race, created life on countless worlds and steered the course of protoss evolution. A number of xel'naga even strove to create the perfect species, one with absolute purity of form. The zerg are their success story, a creation so ravenous and so focused on its purpose - consume, assimilate, expand - that it destroyed its creators.

Using rapid biological evolution and assimilation in place of technological advancement, the zerg Swarm comprises organisms that are the equal of any war machines of the terran or protoss armies. Chitinous plates and razor-sharp claws do the job of steel and lasers, and from the smallest zergling to the mightiest ultralisk, each member of the zerg Swarm is a perfect engine of destruction - a true example of the xel'naga ideal of purity of form.

The zerg leaders are the Overmind, the collective consciousness of the entire Swarm, and Sarah Kerrigan, the Queen of Blades. The Overmind continues to follow its prime imperative. Having learned of the existence of the psionic protoss and judged them to be the ideal race to infest and thereby augment the Swarm's might, the Overmind has long been in search of a race with a trace of psychic ability it could assimilate and evolve. It has found its answer in the terrans. Having infested and evolved psychic Sarah Kerrigan into a new form, the Overmind finally created a creature it could not control. Determined to gain control over the Swarm, Kerrigan lashes out at all who stand in her way.