Blizzard has revealed the details of its first expansion pack for 2010’s top-rated strategy game, StarCraft II. Heart of the Swarm will continue the sci-fi story, introduce a new campaign and add bonus content to the multiplayer.

Publisher Activision-Blizzard announced the strategy sequel would be split into three different games way back in 2008. The first game, which finally hit PC last year, was called Wings of Liberty and focused its attention on an exiled faction of the human, or “Terran,” race.

Heart of the Swarm will follow the story of “Queen of the Blades” Sarah Kerrigan, and put the focus on the Zerg — a race of insectoid aliens obsessed with assimilating other races to steal their genetic mutations. Finally, the trilogy will be completed when the Protoss-themed Legacy of the Void is released.

Swarm will introduce about 20 new single-player missions, with units and abilities that are only available in the campaign. Befitting of the Zerg’s genetic obsession, you’ll be able to evolve your swarms of insects to gain new abilities, subspecies, genetic mutations and evolutionary offshoots.

For example: You could create an evolutionarily superior zergling that can hatch into a batch of broodlings upon death, or a Raptor zergling that can leap short distances. Between missions, you’ll be able to mutate your different zerg species along different strains.

On its website, Blizzard teases that the expansion set will include new units and new maps, but is staying mum about the exact details until Blizzcon (its own gaming expo, held in October 2011). The developer also wants to launch the StarCraft Marketplace, where players can easily find, rate and sort through custom-made levels.

You’ll need Wings of Liberty to play Heart of the Swarm, but Blizzard says the price will reflect the game’s limited content and expansion pack branding. As for the game’s release date — Blizzard is using its tried-and-tested company line. “We will take as much time as is needed to ensure that Heart of the Swarm meets the expectations of our players, as well as our own high standards.”

To see Heart of the Swarm screenshots, visit Wired UK’s gallery.