Starcraft 2 Zerg History

Through xel'naga proto-genetic manipulations, the zerg survived the torrential firestorms of their world and thrived. The zerg developed the ability to burrow into the spinal fluids of less vulnerable creatures on Zerus, parasitically merging with them. These new bodies would then be used to manipulate their surroundings. The zerg began to assimilate the genetic strains and processes of these host creatures. However, the zerg had an undeviating drive to consume only the most advanced species they encountered. Lesser species were eradicated. The xel'naga were surprised to find how quickly the zerg could steer the evolution of their host creatures. They developed armor piercing spines, razor-sharp limbs, and ultra dense carapaces.

Fearful of the chaotic in-fighting that had enveloped the protoss, the xel'naga created the Overmind, an enormous brain-like entity, to bring order to the zerg. At first it was only a semi-sentient entity created from the instincts and collective sentience of the zerg, but it quickly evolved. The Dark Voice undermined the xel'naga's intent; it implanted an overriding directive into the Overmind causing it to seek the violent destruction of the protoss. The Dark Voice's sought to dominate all creation. The Overmind was opposed to the Dark Voice and its goals, but was unable to resist.

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In StarCraft, the Zerg are obsessed with the pursuit of genetic purity, and are the focus of the game's second episode. With the Xel'Naga-empowered Protoss targeted as the ultimate lifeform, the Zerg invade the Terran colonies in the Koprulu Sector to assimilate the Terrans' psionic potential and give the Zerg an edge over the Protoss. Through the actions of the Sons of Korhal, the Zerg are lured to the Confederate capital Tarsonis, where they capture the psionic ghost agent Sarah Kerrigan and infest her. Returning to the Zerg base of operations on Char, the Zerg are attacked by the dark templar Zeratul, who accidentally gives the location of the Protoss homeworld Aiur to the Zerg Overmind. With victory in sight, the Overmind launches an invasion of Aiur and manifests itself on the planet. However, at the end of the game, the Protoss high templar Tassadar sacrifices himself to destroy the Overmind, leaving the Zerg to run rampant and leaderless across the planet.

The Zerg return in Brood War initially as uncontrolled indiscriminate killers. Through the early portions of Brood War, Sarah Kerrigan is at odds with the surviving cerebrates, who have formed a new Overmind to restore control of the Swarm. Through allying herself with the Protoss, Kerrigan strikes at the cerebrates, causing disruption of their plans. Eventually, the UED fleet takes control of Char and pacifies the new Overmind with drugs, putting the cerebrates and most of the Zerg under their control. Kerrigan retaliates by allying with the Dominion and the forces of Jim Raynor and Fenix, regaining the ground lost to the UED. However, she later betrays this alliance, dealing long-term damage to her allies' infrastructures. Proceeding to blackmail Zeratul into killing the new Overmind, Kerrigan's forces destroy the remnants of the UED fleet, giving her full control of the Zerg and establishing the Swarm as the most powerful faction in the sector.

The backstory for the series describes the original Zerg as small larvae, the "most insignificant lifeform" on their homeworld Zerus. The Xel'Naga are responsible for manipulating the evolution of the Zerg, allowing the larval Zerg to parasitically merge with other creatures, taking control of their host's nervous systems. From here, the Zerg can mutate the host and add its DNA to the Zerg gene pool for other Zerg larvae to mutate into. This method of assimilation means that there is no distinctly typical Zerg; rather, many different strains and breeds of Zerg, each based on the original core genus. However, the style of the Zerg is consistent, with breeds using a carapace for armor, and using spines, claws and acids for attacking enemies. Each strain serves a distinct role and possesses the traits necessary to fulfill its function. Zerg colonies also exhibit similar roles, with each structure built within a colony essentially making up an organ in a larger lifeform. Zerg colonies produce a carpet of bio-matter referred to as the "creep", which essentially provides nourishment for Zerg structures and creatures. The Zerg are also shown to be highly dependent on their command structure: if a Zerg should lose its connection to the hive mind, it may turn passive and incapable of action, or become completely uncontrollable and attack allies and enemies alike.

Zerg buildings and units are entirely organic in-game, allowing every Zerg to slowly regenerate if they are damaged. Zerg production is far more centralized than with the Terrans and Protoss; a central hatchery must be utilized to create new Zerg, with other structures providing the necessary technology tree assets, whereas the other two races can produce units from several structures. Zerg units tend to be weaker than those of the other two races, but are also cheaper, allowing for rush tactics to be used. Some Zerg units are capable of infesting enemies with various parasites that range from being able to see what an enemy unit sees to spawning Zerg inside an enemy unit. In addition, Zerg can infest some Terran buildings, allowing for the production of special infested Terran units.

The Zerg are a collective consciousness of a variety of different races assimilated into the Zerg genome. The Zerg were originally commanded by the Zerg Overmind, a manifestation of this hive mind, and under the Overmind's control the Zerg strove for genetic perfection by assimilating the favorable traits of other species. After a species has been assimilated into the Swarm, it is mutated towards a different function within its hierarchy, from being a hive worker to a warrior strain. StarCraft's manual notes that some species bear little resemblance to their original forms after just a short time into assimilation. The Overmind controls the Swarm through secondary agents called cerebrates. Cerebrates command an individual brood of Zerg, each with a distinct tactical role within the hierarchy. Cerebrates further delegate power through the use of overlords for battlefield direction and queens for hive overwatch.

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The vast majority of the Zerg do not have any free will as they are genetically forced to obey the commands of those further up the Zerg hierarchy, although they are sufficiently intelligent to form strategies and work as a team on the battlefield. Despite this, the average Zerg has no sense of self preservation. Along with the Overmind, the cerebrates are the only Zerg with full sapience, each with its own personality and methods, although they too are genetically incapable of disobeying the Overmind. The Overmind also possesses the ability to reincarnate its cerebrates should their bodies be killed, although Protoss dark templar energies are capable of disrupting this process. If a cerebrate is completely dead and cannot be reincarnated, the Overmind loses control of the cerebrate's brood, causing it to mindlessly rampage and attack anything. As a result of the Overmind's death in StarCraft and the subsequent destruction of a new Overmind in Brood War, Blizzard Entertainment's Chris Metzen revealed that the surviving cerebrates perish as they are genetically incapable of surviving without their master, leaving the infested Terran and prize of the Overmind, Sarah Kerrigan, in complete control over the Swarm.

The zerg were "created" by the xel'naga on Zerus. The xel'naga were searching for a counterpart to merge with the protoss, gifted with the purity of form, and continue their lifecycle. In the zerg they found a worthy candidate for the purity of essence. To prevent a similar fragmentation and regression experienced by the protoss during the Aeon of Strife, the xel'naga subordinated all the zerg to a single entity: the Overmind. The ancient species attempted to keep their presence a secret from the Swarm, but eventually the Overmind became aware of them and ordered the zerg to attack. The xel'naga were driven from Zerus, and the zerg gained knowledge of the protoss.

The Overmind believed assimilating the protoss would make the zerg "perfect", and directed the Swarm into space in search of Aiur, the protoss homeworld. Along the way it assimilated multiple species in preparation for the confrontation. As the Swarm neared the Koprulu Sector and the volume of space under protoss stewardship, they discovered the terrans. Of the species encountered, the terrans were unique in having psionic potential. With such abilities the zerg would be able to combat the protoss on even terms.

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The Swarm started the Great War by invading the terran worlds, but were met by a protoss counter-attack. The protoss were aware of the zerg's xel'naga origins and, seeing the effects of zerg infestation, resolved to prevent any zerg foothold. Protoss Executor Tassadar, leading the Koprulu Expeditionary Force, was reluctantly forced to purify numerous terran worlds. Even so, the zerg were not stopped.