The Halo video game and media franchise takes place in a fictional science fiction universe. In the distant past, a race known as the Forerunners fought the parasitic Flood. The Forerunners ultimately activate weapons of mass destruction, the Halo Array, in order to starve the Flood and stop the threat. By the 26th century, humanity, led by the United Nations Space Command (UNSC), is caught in a war with an alien coalition known as the Covenant. The Covenant worship the Forerunners as deities, mistakenly believing that the Halos are a tool of salvation, not destruction. 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 con 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 later games' development. Reception of Halos factions has generally been positive. The Covenant were praised by reviewers as exciting and challenging enemies, and later games featuring the Forerunner Prometheans as enemies were criticized as not living up to the same standard. Characters, weapons, and vehicles of all the factions have been released as toys or promotional materials.
Throughout much of Bungie's development of Halo: Combat Evolved, very little concrete story details had been developed for the story campaign, and what enemies the player character would face. Writer Joseph Staten and other Bungie staff came up with the idea of a coalition of alien races, subsequently deciding that the faction would be motivated by religion. Early in the game's development, environmental artist Paul Russel solidified the concept of three "schools" of Halo architecture for the main factions in the game – the humans of the United Nations Space Command (UNSC), the alien alliance of the Covenant, and the Forerunner structures on which most of the game takes place.