StarCraft: Brood War is the expansion pack for the military science fiction real-time strategy video game StarCraft. Released in December 1998 for Microsoft Windows and June 1999 for Mac OS, it was co-developed by Saffire and Blizzard Entertainment. The expansion pack introduces new campaigns, map tilesets, music, extra units for each race, and upgrade advancements. The campaigns continue the story from where the original StarCraft ended. Brood War takes place in the StarCraft universe, set around the early 26th century. Terran exiles from Earth have colonized a distant area of the Milky Way galaxy called the Koprulu Sector, having established several governments. Eventually, a civil war breaks out and ends with the formation of the Terran Dominion. However, humanity soon becomes caught in a war between the Protoss and the Zerg, which culminates at the end of StarCraft with the death of the Zerg leader, the Overmind, on the Protoss homeworld of Aiur. Without the Overmind to command, the Zerg rampage mindlessly across Aiur, while the cerebrates—the secondary commanders of the Swarm—attempt to regain control. After the discovery of alien life in the Koprulu Sector, the United Earth Directorate (UED)—the international body governing Earth—decides to send an expeditionary force to secure the sector and prevent the aliens from finding Earth. Brood War begins two days after the conclusion of StarCraft. StarCraft focuses around three distinct interstellar species: the psionic Protoss, the adaptable Terrans, and the insectoid Zerg. The game revolves around players collecting resources to construct a base, upgrade their militaries, and ultimately conquer opponents. Brood War's gameplay remains fundamentally unchanged from that of StarCraft, although it made small alterations to unit costs and some abilities, and added some new units. These changes make rushing tactics—a factor that gained some criticism in the original StarCraft—less practical. The single-player campaign has an increased difficulty; missions are no longer entirely linear, and a greater focus on strategy is needed to complete missions. In addition, the game's artificial intelligence (AI) has been augmented so that AI-controlled players are more intelligent and tend to use tactics more effectively. Brood War introduces seven new units. Each race is given access to a unique ground unit: the Zerg can create a defensive unit that can attack from the concealment of its burrow, while the Terrans can train combat medics. The Protoss are able to produce dark templar units, a powerful cloaked melee unit only given to the player in special missions of StarCraft. Protoss players can merge two of these units to create a special spellcaster unit. Each race is also given access to a dedicated air-to-air attack unit.