The Zerg are an invasive hive-minded alien species and the main antagonists of the StarCraft real time strategy video game, as well as one of the three main antagonist factions of the StarCraft franchise as a whole alongside the Terran Dominion and the Forces of Amon. They are one of the dominant powers of the StarCraft universe and one of the three that took part in the Great War, alongside the Terrans and Protoss.
The Zerg are a parasitic hive-mind bent on the assimilation or destruction of all non-Zerg species. Because of their vast numbers and intelligent collective mind, the Zerg are able to invade and occupy numerous planets throughout the galaxy, assimilating their species to enhance themselves. Their ultimate objective is to assimilate the Protoss and become "perfect"; however, due to their psionic abilities a Protoss will naturally reject Zerg physiology and thus Zerg-Protoss hybridization cannot occur naturally.
The Zerg species first matured on the planet Zerus. They were eventually discovered by Amon and his fallen Xel'Naga after they abandoned the Protoss. Amon and his followers intervened in the Zerg's evolution to give them "purity of essence", in contrast to the Protoss' "purity of form", leading to the formation of the Zerg Swarm. To control the Zerg Swarm's hive mind, the Overmind was created. The Zerg that avoided having their genetics tampered with by Amon became known as "Primal Zerg" and hid to avoid being assimilated into the Swarm.
When the other Xel'Naga caught wind of Amon's actions and attempted to stop him, Amon used the Zerg to exterminate the other Xel'Naga. However, in the process of assimilating the Xel'Naga, the Zerg became aware of the previous species the Xel'Naga had genetically altered, the Protoss, and began to desire to assimilate them to achieve genetic perfection. However, the Overmind, aware of Amon's destructive goals, began secretly searching for a way to subvert Amon's control over the Zerg and free the Zerg from the fallen Xel'Naga's grasp.
First Great War
After having folded multiple species into their brood, the Zerg launched an invasion of Terran worlds with the purpose of assimilating the Terrans into the Swarm, sighting their psionic potential as a way to counter the psionic powers of the Protoss. This led to the massive conflict known as the "Great War" breaking out between the Protoss Empire, Terran Confederacy and the Zerg Swarm.
During the Great War, the rebel group known as the Sons of Korhal, led by Arcturus Mengsk, exploited the Zerg invasion for their revolt against the Terran Confederacy. Using Psi-emitters, Mengsk attracted the Zerg to attack Confederate bases, weakening the Confederacy's power and inadvertently assisting Mengsk in his overthrow of the Confederacy to establish the Terran Dominion. When the Sons of Korhal launched an invasion of the Confederate capital of Tarsonis, Mengsk used psi-emitters to draw the Zerg to the planets surface and attack the planet's population. During the battle Sarah Kerrigan was sent to hold off the Protoss from attacking the Zerg so they could wipe out the Old Families that ruled the Confederacy, leading to her being captured and assimilated into the Zerg. Kerrigan was transformed into the Zerg's "Queen of Blades" and was positioned by the Overmind to succeed it as leader of the Zerg, which would free the Zerg from Amon's control.
The Overmind was eventually destroyed on Aiur by the Protoss, halting the Zerg's advancement and bringing the Great War to a close.
With the Overmind destroyed, a succession crisis erupted between Sarah Kerrigan and the Overmind's chief Cerebrate, Daggoth, which sought to install a second Overmind as ruler of the Zerg. This crisis led to a civil war within the Zerg known as the "Brood War" between Kerrigan's Zerg and Daggoth's renegade Zerg which eventually drew in numerous other outside factions.
The United Earth Directorate soon intervened in the war and seized the second Overmind, desiring to use the Zerg to aid in their campaign against the Terrans. To combat the opposing Zerg, the UED utilized Psi Disruptors seized from the defunct Confederacy to block psionic communications between the Zerg and hinder their ability to coordinate. In response, Kerrigan was forced to ally with the factions opposing the UED and used them to destroy the Disruptors. After slaying the second Overmind, Kerrigan brought the entire Zerg collective under her control and exterminated the remaining Cerebrates so they could be replaced by the autonomous Brood Mothers. The Terrans, Protoss and remnants of the UED all launched a joint campaign against the Zerg to stop Kerrigan but failed, leading to the conflict ending in an effective stalemate.
The Zerg are a hive collective governed by a central authority known as the Zerg Overmind. The Overmind issues orders to the different Zerg broods through Cerebrates, which share a consciousness with the Overmind and thus they are incapable of disobeying. The Cerebrates in turn, act as generals of the Swarm and each command a different brood of the Zerg.
After Kerrigan took over the Zerg, the Zerg became less of a hive mind. The Cerebrates were destroyed and replaced with the Brood Mothers, which each shared an autonomous consciousness from Kerrigan and thus possessed free will.