Blizzard unveiled Heart of the Swarm the latest chapters for its popular Real-Time Strategy game, StarCraft 2: Wings of Liberty.

The expansion adds 20 new missions to the single-player campaign as well as three new units to each race. In the new campaign players will mostly control the race known as the Zerg, a species of bug-like aliens known for their tendency to swarm enemies.

StarCraft is a strategy game where gamers choose between three playable races, the human-like Terrans, the techno-and-psionic powered Protoss and the Zerg. Though the game has a single-player campaign many gamers play the game for its multiplayer content where they collect resource to build armies and use those armies to wipe each other out.

The Heart of the Swarm story focuses on Sarah Kerrigan, a former Terran solider who was transformed into the Queen of Blades, leader of the Zerg Swarm, in a previous game. She is transformed back into a Terran in this campaign as she seeks to regain her alien thrown.

Kerrigan will be a playable hero in the game, giving unique abilities from other units and getting upgrades throughout the campaign. Though she has been dethroned, Kerrigan still rules over a small brood of zerglings. Through the campaign players will also be able to evolve strands of zerglings, giving them the ability to reproduce quicker or leap short distances into battle.

The expansion also adds a few units to each race of the game. The Protoss get the Replicant, Oracle, and Tempest. The Terran receives the Warhound, Battle Hellion and the Shredder. And the Viper and Swarm Host are added to the Zerg.

This is the first expansion the StarCraft 2 franchise. The first game focused on a Terran campaign. The second expansion will be called Legacy of the Void following a Protoss driven story. No word on when the game will come out or how much it will cost when it does.