Blizzard has confirmed rumours that its StarCraft II expansion pack Heart of the Swarm will miss the Christmas sales period, with its launch now delayed until some time in 2013.

The Heart of the Swarm expansion was originally supposed to add seven new units to the multiplayer game and remove three existing units for balancing purposes. Closed beta testing caused Blizzard to change its plans, however, dropping the planned Shredder and Warhound unit additions for the Terrans due to balancing issues and replacing them with Widow Mines - variants on Spider Mines that fire splash-damage missiles - and tweaks to the Hellion, Ghost, Battlecruiser and Reaper units.

Protoss players will get three of the planned four new multiplayer units, the Oracle, the Mothership Core and the Tempest, while the Replicant has been ditched over fears it would encourage players to 'spam' masses of identical units. Other new units found in the expansion include the Overseer, Viper and Swarm Host, the latter of which spawns units known as Locusts.

The single-player game also sees some modifications, with an expanded plot taking over where Wings of Liberty ended. If you haven't played the Wings of Liberty missions, by the way, you should probably skip the rest of this paragraph. Designed to focus on the Zerg race and the exploits of Sarah Kerrigan, the expansion campaign is to start with Kerrigan attempting to unite the fractured Zerg factions and assume leadership of the swarm in order to exact revenge on Terran Emperor Arcturus Mengsk.

The biggest changes, however, come to the game's suitability for competitive play. Under the new expansion, StarCraft II will gain a new player progression system, support for custom clans, and resumable game replay functionality.

Sadly, all these features will now have to wait. While the company had originally planned to get the expansion out by the end of the year, some last-minute delays mean it will now be out some time in 2013. While no firm date has been set, Blizzard chief executive Mike Morhaime told investors that it should be released in the first half of 2013 during the company's latest earnings call.