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Starcraft is a multimedia franchise created by Blizzard Entertainment. Hundreds of years into the future the rulers of Earth decided to start a colonization project, and chose as its colonizers its criminals, political enemies, and other undesirables. The colony ships reached their target on the far side of the galaxy: a sector of barely inhabitable planets. Still humanity was able to survive, spreading to new systems and warring amongst themselves. That was before the Zerg and Protoss appeared.
The Zerg are a galactic pestilence, devouring and stripping planets bare before moving on. The Protoss, who were genetically manipulated by the same antediluvian race that created the Zerg, made it their mission to wipe out their sibling species. This ended up putting them in conflict with Humanity when the primary government of the colonized worlds found out they could turn the Zerg into a weapon through Psionic emitters. A rebel force stole this technology and, with the assistance of Ghost Sarah Kerrigan, used the emitter on the Colonial homeworld. Sarah is unable to escape the planet in time, and is swarmed by the Zerg.
For better or worse Sarah is not killed in the attack, but is instead cocooned and slowly transformed into a Zerg. The Overmind of the Zerg saw her psychic powers as being a great asset to the swarm, and once she hatches from her chrysalis the new Kerrigan is more than happy to serve. The Overmind continues to dominate its rivals, invading even the Protoss homeworld of Auir itself. With the use of forbidden techniques, however, the Protoss are able to strike the Overmind and kill it permanently. The loss of the Overmind creates a power vacuum in the Zerg with Kerrigan swiftly fills.
Christening herself the Queen of Blades, Kerrigan wages war on the Colonial humans, the scattered Protoss, and even an armada of ships which have arrived from Earth. The Earth forces try their own hand at creating an Overmind under their control, but Sarah's machinations end with all three forces decimated and the Earth fleet totally wiped out. She then withdraws from the sector and makes plans in secret.