"Rule number one for running an empire, son: When the Zerg are a topic of conversation, the niceties go out the airlock."- Arcturus Mengsk(src)
The Zerg Swarm is a terrifying and ruthless amalgamation of biologically advanced, arthropodal aliens. Dedicated to the pursuit of genetic perfection, the zerg relentlessly hunt down and assimilate advanced species across the galaxy, incorporating useful genetic code into their own.
Alongside the protoss and terrans, the zerg stand as one of the three dominant species of the Milky Way and are universally feared, hated and hunted as such.The Swarm in itself makes up a third of the power of the Koprulu Sector.
Creation of the Swarm
The xel'naga discovered the original zerg on Zerus.The xel'naga gifted the zerg with the purity of essence, making them the counterparts of the protoss, gifted with the purity of form; the zerg and protoss were intended to eventually merge to perpetuate the xel'naga lifecycle. At this point the zerg species diverged into two distinct branches; the Zerg Swarm and the primal zerg.The xel'naga created the Overmind to control the zerg swarm and prevent a repeat of the protoss' Aeon of Strife. The Overmind oversaw the zerg swarm's rapid evolution, created the cerebrates, and divided the Swarm into broods.The primal zerg, meanwhile, went into hiding to preserve their individuality.
The Overmind's purpose was subverted by the fallen xel'naga named Amon, who would become known as the Dark Voice; the Swarm drove away the xel'naga, and embarked on an interstellar journey toward Aiur, the protoss homeworld, to forcibly assimilate the protoss. The Overmind was aware, but unable to resist Amon's influence. The Overmind knew Amon's ultimate goal would destroy the zerg.
Amon sponsored his own protoss-zerg hybridization program. The first hybrids were ready by the end of the Brood War.
The Great War
Having assimilated multiple species and evolved during the journey, the Swarm arrived at the Koprulu Sector near the Protoss Empire. The Overmind ordered the assimilation of the terrans; their psionic potential was needed to fight the protoss. The protoss detected the zerg, learned of their xel'naga origins, and dispatched the Koprulu Expeditionary Force under Executor Tassadar to deny the zerg a foothold.
The Great War broke out with the zerg invasion of the terran worlds. Protoss purification, and terran internal strife, failed to contain the invasion. The terran rebel group, the Sons of Korhal, used and manipulated the invasion to overthrow the Terran Confederacy and form the Terran Dominion.The zerg captured Sarah Kerrigan at the Battle of Tarsonis and withdrew to Char to transform her into the Queen of Blades. The Overmind intended Kerrigan to succeed it as the Swarm's leader, freeing the zerg from the Dark Voice. It had no concern over the other races of the galaxy.
The zerg were attacked on Char by the protoss; a renegade Tassadar joined forces with Prelate Zeratul, a Dark Templar, on Char. Zeratul, wielding the Dark Templar's power of the Void, permanently slew Zasz, the chief cerebrate. This created a brief mental link between the Prelate and the Overmind, through which the Overmind learned the exact location of Aiur. Kerrigan remained on Char while the Swarm invaded Aiur.
Resistance against the zerg invasion was hampered by the protoss Conclave's overconfidence and a short protoss civil war.[14] The planet was devastated.[15] The Overmind relocated to Aiur, where it was slain by the protoss. The protoss subsequently withdrew to Shakuras, the Dark Templar homeworld.