The Zerg are a terrifying, ruthless, biologically advanced xenomorphic race in a science fiction game called “Starcraft”. Dedicated to the pursuit of genetic perfection, this species hunts down any advanced species that could provide useful genetic code into their own, and exterminate any species that might corrupt the purity of the Zerg.
The Zerg were created by the Xel’naga on a world known as Zerus that has highly unstable volcanic activity. The planet Zerus is situated in the central regions of the Milky Way. The Xel’naga was searching for a compatible race to merge with the Protoss to be gifted with purity of form and continue their life cycle. In the Zerg they found a worthy purity of essence. To prevent a similar fate as the Protoss experienced during the Aeon of Strife (Protoss Great War) the Xel’naga made the entire Zerg race follow one leader, the Overmind. However the Overmind itself was controlled by a being known as the “Dark Voice”. The being tried to keep their identity a secret from the swarm but sooner or later the Zerg found out about their presence and ordered the Zerg to attack. The Xel’naga was driven from Zerus, and the Zerg gained knowledge about the Protoss.
The Dark Voice that controlled the Overmind forced it to seek out the Protoss. The Overmind believed assimilating the Protoss with Zerg would make the swarm perfect. The Zerg then set out in space in search of Aiur. Along the way it assimilated multiple species and near the koprulu sector they discovered the Terrans, aka Humans, that were sent to colonise in the Koprulu sector and lost contact with earth. Of the species encountered, the Terrans were unique with psionic potential. With such ability the Zerg would be able combat the Protoss on even terms.
The Swarm started the Great War by invading terran worlds, but were met with a counter attack by the Protoss. The Protoss, aware of the Zerg’s Xel’naga origin and, seeing the Zerg infestation, resolved to prevent any Zerg foothold. Protoss Executor Tassadar leading the Koprulu expeditionary force, was just able to purify (in Protoss terms meaning “destroy”) some of the Terran worlds, but still the Zerg was not stopped.
The Great War also sparked a large scale terran civil war. Terrans on both sides of the civil war (confederacy and the dominion) wanted to use the Zerg for their own advantage. The rebel Sons of Korhal manipulated the Zerg through psi-emitters, drawing the Swarm onto the defenses of the failing Terran Confederacy, first at Antiga Prime and then at the Confederate capital world of Tarsonis. At Tarsonis, the Zerg not only delivered the killing blow to the Confederacy, but also captured Sarah Kerrigan, a psionic ghost operative. The Overmind believed Kerrigan was the key to stopping the Dark Voice. The Swarm withdrew to Char to complete the infestation of Kerrigan into the Queen of Blades. She was the Overmind's way to resist the Dark Voice’s control of the Swarm.
Kerrigan drew the Protoss leaders Tassadar and Zeratul to the Zerg home world of Char. The latter was a Dark Templar with the ability to permanently slay cerebrates. Striking an alliance, the two Protoss groups fought repeated battles against Zerg forces led by Infested Kerrigan. Zeratul slew Zasz, a Zerg cerebrates, somehow accidentally giving the Overmind the location of Aiur. The majority of the Zerg Swarm quickly traveled to Aiur, leaving Kerrigan behind with a small number of Zerg to fight the Protoss still on Char. The Zerg, now on a destroyed homeworld of the Protoss Aiur, enjoyed victories over the Protoss.
The Protoss on Char eventually escaped to Aiur, where they defeated the Overmind. The death of the Overmind caused the temporarily leaderless Zerg to rampage; most of Aiur's population was killed. The Zerg split into two factions, one led by Daggoth (the second Overmind) and the other led by Infested Kerrigan. The Protoss fled to Shakuras home of the Dark Templar followed by Daggoth's Zerg. The Protoss collected a pair of artifacts in order to destroy the Zerg and make their new home safe, while forming an unexpected alliance with Kerrigan. The Protoss destroyed the Zerg on Shakuras in order to protect themselves, realizing that Kerrigan was persuading them to fighting on her side in the Zerg civil war.
The arrival of the United Earth directorate from earth moving into the Koprulu sector to establish new colonies, due to the over populated planet earth, upset Kerrigan’s plans. After the UED (United Earth Directorate) conquered the terran dominion and chased after the emperor (Arcturas Mengsk) they took control and activated a powerful anti-Zerg weapon, the psi disruptor. Samir Duran, a UED commander, used the technology and overtook the second Overmind turning most of the Zerg to his side while weakening Kerrigan’s control over her own Zerg. Kerrigan was weakening and desperately struck an uneasy alliance with Arcturas Mengsk and JIm Raynor. After the alliance had weakened the UED force, Kerrigan betrayed them. Samir Duran had betrayed the UED and sided with Kerrigan. She turned her attention to the Protoss and kidnapped Dark Templar leader Raszgal, forcing Zeratul to slay the second Overmind. Now Kerrigan was leader of all the Zerg broods but she was suddenly abandoned by Samir Duran.
Duran worked on a mysterious Protoss/Zerg hybridization project which Zeratul discovered. Meanwhile the Protoss, UED and Terran Dominion, all converged on Kerrigan but she was able to defeat them.
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 any hostile actions against the terrans and Protoss for four years. No expeditions sent to Swarm-controlled worlds have come back alive. The Swarm started the Second Great War by attacking terran space in search of a Xel’naga device. The Zerg were beaten to the device by Raynor's Raiders. Char was invaded by a terran battlegroup that used the device to deinfest Kerrigan. Most of Kerrigan's forces had yet to return to Char on their leviathans and died fighting each other in the void. Many of Kerrigan's brood mothers, sensing her weakness, moved their broods throughout the Koprulu Sector. Kerrigan swore to recover them.