The history of the zerg spans an unknown lengthy period of time.
Relations with the Xel'Naga
The original zerg species was discovered on the ash world Zerus, in the center of the Milky Way Galaxy, by the xel'naga, a mysterious and ancient race of extragalactic geneticists, after they had abandoned their first experiment, the protoss, on Aiur. The zerg organism was a small, larval organism, but it contained the purity of essence sought by the xel'naga, whose complicated life cycle required selecting and evolving species with purity of form and essence and having them naturally merge into the next incarnation of the xel'naga over eons.
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 Overmind did not directly relay orders to its minions. As new species were added to the Swarm, it began to relay orders through cerebrates, giant versions of the zerg larvae. Direct orders were relayed through queens and overlords.
Assault on the Xel'Naga
The Overmind began to think of its future. In order to continue finding new species, it had to leave Zerus. Extending its senses into space, it became aware of enormous space-faring organisms. It lured them to Zerus with its psychic abilities, then assimilated them into the Swarm as behemoths. Soon, all the zerg had the ability to survive in space. The xel'naga considered this a triumph. The zerg had not only survived their weaknesses, they had retained the purity of their terrible overriding essence.
The Overmind extended itself further into space, becoming aware of the xel'naga worldships in orbit above Zerus. The xel'naga realized their psionic link with the Overmind had been severed – the Overmind effectively hid itself from their view.
The Overmind sent its minions into space, attacking the unsuspecting xel'naga. Despite the ancient race's best efforts, it was overwhelmed as the zerg crashed into the hulls of its ships. Within a few hours the zerg laid waste to the xel'naga fleet. The greater whole of the xel'naga were consumed by the raging, genetic whirlwind of the zerg, and the Overmind gained their knowledge and insights. Processing thousands of sentient beings into itself caused the Overmind to grow much more powerful. It learned the secrets of the sacred khaydarin crystals, and began to incorporate the energies of these crystals into its own. Through the knowledge gained from the xel'naga, the Overmind was able to increase the level of sentience in many of the higher zerg strains, while still keeping them fully under its control.
The Overmind dissected the memories of the xel'naga experiments with the protoss. The zerg then devoted their energies to seeking out the powerful protoss in the hope of absorbing them into the Swarm, uniting purity of form and of essence, to create a totally "perfect" being.
The defeat of the xel'naga threw their plans for their life cycle into turmoil.
The zerg did not know the exact location of Aiur, homeworld of the protoss, but they were able to travel towards it, adding new species (such as the mutalisk) to the Swarm along the way. However, the Overmind despaired; how could it defeat the protoss with their powerful psionic abilities, when its own were so limited?
In the shadow of the protoss lay the Koprulu Sector, a patch of space inhabited by the terrans. This species had been discovered by zerg deep space probes. Normally the zerg would not be interested in this species, believing them to be weak, but they exhibited a powerful psionic potential. Their probes discovered humans living on over a dozen worlds.
Invasion of the Koprulu Sector
The zerg slowly made their way towards the terran worlds. The journey lasted for sixty years, but eventually the massive, extended Zerg Swarm reached the outskirts of the Koprulu Sector. A number of Fringe Worlds were immediately infested.
The zerg continued to penetrate into the Sector. The Terran Confederacy colony Chau Sara was subtly invaded first with hive spores, before the bulk of the zerg force arrived to complete the infestation process. The Confederacy attempted to keep the zerg presence a secret from its own citizens.
The terrans were defeated. A protoss fleet led by Executor Tassadar arrived shortly thereafter and sterilized the planet, effectively ending the infestation. The Overmind ordered its minions not to resist so that it could observe the protoss in action.
The zerg were lured to Mar Sara with psi emitters. The Confederates made little attempt to oppose them, even arresting militias which dared to attack them. Meanwhile, the zerg invaded several other worlds, which were then purified by the protoss.
A New Agent
The Sons of Korhal's leader, Arcturus Mengsk, unwittingly played a role in the Overmind's designs. The zerg devastated the Confederacy's power on Tarsonis, trapping the survivors on the planet. The protoss Expeditionary Force under Tassadar arrived, intending to destroy the zerg infestation. Mengsk ordered his subordinate, a ghost named Sarah Kerrigan, to lead his forces against the protoss. He feared the Confederates would escape, so he wanted the zerg to remain in place to keep them there. Kerrigan reluctantly followed his orders. She defeated the protoss, but was overwhelmed by a wave of zerg. Mengsk made no effort to save her, as she had become a liability.
The zerg captured Kerrigan, but rather than killing her, they placed her within a chrysalis to infest her. In Kerrigan the Zerg knew she was needed to prevent a horrible future which both Protoss and Zerg are destroyed by the hybrids. With their prize in hand, the zerg partially withdrew from terran space back to Char where Kerrigan was reborn into the Zerg Swarm as the powerful Queen of Blades. Unfortunately for the zerg, her psionic emanations drew the attention of several enemies; General Edmund Duke of the Terran Dominion, Raynor's Raiders, the remnants of Tassadar's Expeditionary Force, and the Dark Templar.
Breakthrough on Char
Kerrigan broke her psionic conditioning using Dominion Ghost Program files, and with her new powers she detected the presence of Tassadar and his protoss warriors. She defeated his forces, but his ally, Zeratul, Prelate of the Dark Templar, took advantage of her distraction to kill Zasz, Cerebrate of the Garm Brood. This action harmed the Overmind, preventing it from speaking. The leaderless Garm were eradicated by the zerg.
The protoss had struck a great victory, but it was also their undoing. A brief psychic link had formed between Zeratul and the Overmind at the moment of Zasz's death. The Overmind had been able to read Zeratul's mind, and from him learned the secret location of the protoss homeworld of Aiur. Exultant, the zerg began their preparations for the invasion of Aiur and the fulfillment of their long quest for perfection. Meanwhile, Kerrigan's forces inflicted a revenge assault against the protoss, killing many. Tassadar and some troops were stranded on Char's space platform while Zeratul and his Dark Templar were trapped in an abandoned terran installation.
The Invasion of Aiur
Believing it was winning the war, the Conclave dispatched a force of warriors to Char to arrest Tassadar. Tassadar convinced the force to help him rescue the Dark Templar, the only force capable of truly defeating the zerg. The protoss were able to rescue Zeratul.
Tassadar took the Dark Templar with him to Aiur, immediately triggering a civil war. The Conclave felt the Dark Templar were a greater threat than even the zerg. Tassadar emerged victorious over the Conclave.
Freed from Conclave interference, Zeratul took part in an assault on two zerg hive clusters, slaying two cerebrates. His forces hoped this action would distract the Overmind long enough to directly assault it. Then the protoss, with assistance from the terran Jim Raynor, moved past the thinned zerg defenses to the Overmind itself. The zerg were defeated, and Tassadar, drawing upon the Dark Templar powers he had been taught, sacrificed himself to destroy the Overmind. The Swarm was thrown into disarray and the zerg rampaged across Aiur wrecking senseless death and destruction.
Moments of Transition
With the fall of the Overmind, the zerg on Char fell into infighting. In response, a number of cerebrates led by Daggoth began merging together to form a new Overmind on Char. In the interim this left the Swarm divided between the new Overmind's Renegade Zerg faction and Infested Kerrigan's control. Infested Kerrigan secretly traveled to the Dark Templar homeworld of Shakuras, subtly gaining control over the mind of its Matriarch, Raszagal.
The Brood War
The zerg rampaged over Aiur, forcing the protoss to flee. They traveled through a warp gate to Shakuras. The Renegade Zerg which followed them through were controlled by Daggoth and the second Overmind. The protoss shut down the Gate, at Fenix and Raynor's request, stranding them and other protoss on Aiur.
Two cerebrates made it to Shakuras, and set themselves around its xel'naga temple. The Dark Templar destroyed them, and were surprised by the appearance of Infested Kerrigan. She proposed an alliance; she would help them recover the two crystals which could activate the temple, if they would assist her in destroying the new Overmind growing on Char. Most of the protoss were more interested in saving Shakuras from the Renegade Zerg, but Matriarch Raszagal was willing to listen.
Kerrigan assisted the protoss in collecting the crystals, but when they returned to Shakuras, they discovered that Aldaris had taken command of a legion of warriors and attacked the forces of the Matriarch. Aldaris was defeated, but instead of surrendering, he told Zeratul and Artanis that Kerrigan had been manipulating Raszagal's mind ... Kerrigan interrupted by slaying Aldaris. Zeratul banished her from the world. Harnessing the powers of the crystals, the protoss activated the xel'naga temple, obliterating the zerg on Shakuras. Due to the machinations, the cerebrates which refused to follow Kerrigan's lead were all but destroyed.
To this end they took control of the Psi Disrupter, a former Confederate anti-zerg weapon capable of disrupting zerg communications, despite the machinations of Kerrigan's spy, Samir Duran. The Disrupter weakened Kerrigan's control over her own zerg forces and made it possible for the UED to successfully assault Char, where they captured the second Overmind. The majority of the zerg, in the form of the UED Slave Broods, then fell under the control of the UED.
The United Earth Directorate Expeditionary Force was in a weak strategic position, having to hold Braxis (the world which housed the Psi Disrupter), Korhal (former capital of the Terran Dominion, which it captured) and Char (home of the enslaved Overmind).
Kerrigan struck an alliance with Fenix, Jim Raynor ... and Arcturus Mengsk. In return for her assistance in defeating the UED, they would help her destroy the Psi Disrupter. The allies helped Kerrigan to destroy the Disrupter, and she in turn assisted in a successful assault on the UED forces on Korhal.
Kerrigan and Duran quickly betrayed their former allies however, attacking both as they rested from the previous battle. Fenix himself was slain in combat, along with Dominion General Edmund Duke.
Kerrigan had one more target; the second Overmind. She and Duran planned an invasion of Shakuras in order to acquire Matriarch Raszagal. Kerrigan brought Raszagal to Char and used her as a bargaining chip; she would allow Raszagal to return to Zeratul if his Dark Templar worked alongside her own warriors and slew the second Overmind for her. Raszagal convinced Zeratul to cooperate. Together the forces permanently slew the remaining cerebrates. and the second Overmind, with Zeratul personally delivering the fatal blow. When he demanded the return of Raszagal, the Matriarch refused. Zeratul abducted her, surprising Kerrigan.
Kerrigan's forces hunted down Raszagal. Zeratul was forced to slay her in order to free her from Kerrigan's control. Meanwhile Kerrigan's attacks on the protoss scattered them, at least temporarily.
With these objectives completed and few, if any, opposing cerebrates remaining, Kerrigan became the leader of the Swarm in the sector as the Queen of Blades.
The Dominion, protoss, and UED struck back with unexpected alacrity before the zerg could consolidate. In an epic battle over Char the zerg forced their assailants into ignominious retreat, saving Kerrigan and the Zerg Swarm from retribution. Kerrigan destroyed the retreating UED forces.
The zerg of Aiur are still considered loyal to the Swarm.
- Zerg trailer(src)
After the Brood War the zerg remained on worlds they had already infested, sending out forces to explore and exploit various worlds and assimilate new species, and refrained from engaging in overt hostile actions against the terrans and protoss for four years, only taking part in occassional skirmishes. No expeditions sent to Swarm-controlled worlds have come back alive. UNN and other news services which cover general threats did not report on the zerg during this time.
Any damage from the tumultuous past has been made good in the four years following the Brood War. It is estimated that there are 10 billion zerg situated in 150 primary and 1029 tertiary hive clusters on Char alone. In 2502, the zerg were estimated to have infested over a hundred worlds. According to Infested Kerrigan, the zerg are becoming "much, much more, for the final metamorphosis has only just begun."
Battles on Aiur and Beyond
At least some zerg remained on Aiur. These zerg were initially undirected to the point of killing one another, enabling the remaining protoss stranded on Aiur to coalesce into two groups, the Shel'na Kryhas and the Tal'darim. Some time after the warp gate was disabled, the zerg of Aiur became more intelligent but were still not as vicious or focused as when the Overmind controlled them. They wandered Aiur like "discarded tools" and did not spawn new minions. Furthermore, when Infested Kerrigan's newest infested terran commander, Ethan Stewart, traveled there with a zerg force, he was unable to reinforce himself; his numbers remained limited.
The Swarm has returned to being the destructive force it once was, invading the world of Dead Man's Rock and battling Dominion forces on Aiur in order to recover a protoss preserver, Zamara. Zerg forces then followed the preserver to Ehlna, a Dark Templar holy moon, but were defeated by the protoss and Terran Dominion.
Some time after the Brood War, the feelings of unease the Queen of Blades had felt at the end of the war continued to concern her. Insight gained of the coming threat during the interbellum caused only pessimism, and she was resigned to accept it. Eventually, Kerrigan and her zerg were drawn to the world of Ulaan in search of a prophecy that heralded the return of the xel'naga. There, she confronted Zeratul, who in turn severed her left wing, which she quickly regenerated. She had seen portents of what the future would bring, and already knew enough to believe the future was without hope. Zeratul disagreed, afirming that there was always hope before slipping away. As Zeratul retrieved the prophecy's fragments, the zerg attempted to stop him, but to no avail.
Return of the Swarm
- Main article: Second Great War
Events came to a head when, in 2504, the Swarm invaded Dominion space as a whole, thus starting the Second Great War. With Kerrigan personally leading them, the zerg sought out alien artifacts. One of such, located on Mar Sara, had been already recovered by Raynor's Raiders. The Raiders were forced to hold their position until evacuation, in form of battlecruiser Hyperion, arrived.
The search for the artifacts continued. On Monlyth, the Swarm battled a group of protoss fanatics who guarded one of the artifacts. However, before Kerrigan arrived, the Raiders again sneaked in and grabbed it.
Kerrigan tried again on Tyrador VIII, where the [[Template:Moebius Foundation]] was based. Their own forces had managed to evacuate for the most part, but were unable to destroy their data cores which would reveal the locations of the rest of the artifacts. Raynor's Raiders were hired by the Foundation's leader, Dr. Emil Narud, to destroy the data cores before Kerrigan could get to them. The Raiders were successful in this action.
Battle of Char
- Main article: Battle of Char (Dangerous Game)
While Kerrigan was on Char, an assault force made of the Raynor's Raiders and Valerian Mengsk's fleet attacked the planet. The prince, which had took half the Dominion fleet as well as the services of General Horace Warfield to help him in his endeavour, intended to deinfest Kerrigan using a Xel'naga device built from the artifacts the Raynor's Raiders had previously recovered. This was not an easy task, though, as the Dominion fleet was caught by surprise by a zerg preemptive strike while in orbit. Following Warfield's plan of a frontal assault, Dominion battlegroups attempted to land on the planet's surface and to establish a foothold. However, as most attack groups were either destroyed or scattered it fell to Jim Raynor's hands to gather them and build a ground force able to fight the zerg.
As the battle was going, Kerrigan recalled the forces she had previosuly sent to invade the Dominion space back to Char. With their forces reduced by initial zerg attacks and time against them, Raynor and Warfield agreed to mount a secondary assault aimed at weakening zerg air or ground support.
After the zerg's support was taken out, the terran forces rallied to launch the final attack on the primary hive, after an inspiring speech by Raynor. General Warfield, with a new cybernetic arm, delivered the fully assembled xel'naga artifact to Raynor, where the terran forces had to hold out against the zerg while the artifact charged up. In the aftermath of the battle, the artifact became fully charged and launched a powerful energy nova that destroyed the zerg forces in the vicinity and partially de-infested Sarah Kerrigan.