StarCraft II: Heart of the Swarm won"t be officially released until March 12th, 2013. However, Blizzard has been running a multiplayer beta test for the upcoming RTS expansion pack for a little while now. The beta has been invite only but earlier this month Blizzard opened up the beta to more players.
Specifically, the game"s official website announced that people who have already pre-ordered the expansion via the digital download version on Battle.net on or before December 18th can go ahead and play in the multiplayer beta test right now. If you bought or will buy the expansion after that date, you will still be able to get into the beta test. Blizzard will be adding more players to the beta on a weekly basis. Blizzard also said that retailers like Amazon are also offering promotional codes for entering the beta if people purchase the expansion from their stores.
This is the first of two planned expansion packs for StarCraft II. Heart of the Swarm will also have a single player campaign that will focus on the Zerg faction, but the multiplayer portion will include new content for the game"s other two factions as well.
Source: StarCraft II website | Image via Blizzard