Zerus was the original homeworld of the Zerg, and is an inhospitable planet that is wrought with severely unstable volcanic activity and was home to several species of giant insectoid creatures that were capable of surviving the harsh atmosphere.
The Xel'Naga — an advanced cosmic race that sought to create and care for lesser races — journeyed through the universe to create a race with a "purity of essence". The Xel'Naga settled on Zerus, and chose the most insignificant life-form upon the planet to create their new species: a larval creature known as the Zerg.
They nurtured these creatures, allowing them to affect their surroundings in almost parasitic ways. Eventually, the larvae began infesting the creatures of the planet by burrowing themselves into the spinal cord of the creatures and assimilating their DNA, turning them into monstrosities. The Zerg spread rapidly across the planet, infesting every living life-form on the surface while rapidly growing in numbers.
The Xel'Naga, remembering that their failure with the Protoss was due to their pushing the evolution of a sentient species too quickly, decided to follow a different path with the Zerg. To counter this problem, the Xel'Naga structured the collective sentience of the Zerg into a hive mind controlled by the only fully sentient Zerg: the Zerg Overmind.
However, the pride of the Xel'Naga was their downfall. The Overmind expanded its forces and eventually became aware of the Xel'Naga's presence. In response, the Overmind severed its psychic link with the Xel'Naga and then launched an attack against them. Most of the Xel'Naga race was destroyed and assimilated by the Zerg assault. In this way, the Overmind gained the knowledge of its masters, including their experiments with evolution, proto-genetics, the use of the Khaydarin crystals, and the existence of the Protoss. The Zerg eventually abandoned the planet, leaving it completely devoid of life.
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