The Halo video game and media franchise takes place in a science fiction universe, where there are four major factions players encounter or control. Halo's story has 26th century humanity, led by the United Nations Space Command (UNSC), caught in a war with an alien coalition known as the Covenant. In the 2001 video game Halo: Combat Evolved, the UNSC ship Pillar of Autumn discovers a mysterious ringworld known as "Halo". The massive installation was built by an enigmatic race known as the Forerunners, who have long since disappeared; the Covenant worship the Forerunner as gods. During the course of the game, players discover that the Halos were built as a weapon of last resort against the Flood, an extragalactic parasite which is driven to consume all sentient life. The Forerunners were forced to activate the Halo network, killing themselves and any potential Flood hosts, in an effort to starve the Flood to death. The Covenant leadership discovered Humans could interact with Forerunner technology, and decide to destroy humanity in order to suppress this fact. The Flood, meanwhile, escape the confines of Halo and threaten to spread across the galaxy again.
A large portion of the series' success lies in the creation of a believable world, and Bungie reinforced the fictional factions the player interacts with via a lengthy design process with the release of each game. The overall design of each faction was slowly developed before the release of the first game in the series, Halo: Combat Evolved, and continually refined in the later games' development.
Reception of Halo's factions has generally been positive. The Covenant were praised by reviewers as exciting and challenging enemies. Characters, weapons and vehicles of all the factions have been released as toys or promotional materials.
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... Halos universe and the factions in it have been well-received, both from a storytelling point of view and from game play ... Gamasutra lauded Halos artificial intelligence ... allied UNSC characters reviewers noted that in Halo3, they were often more of a hindrance than help ...