Halo is a military science fiction franchise created by Bungie and now managed and developed by 343 Industries a subsidiary of Microsoft Studios. The series centers on an interstellar war between humanity and a theocratic alliance of aliens known as the Covenant. The Covenant, led by their religious leaders called the Prophets, worship an ancient civilization known as the Forerunners, who perished in combat with the parasitic Flood. The central focus of the franchise builds off the experiences of Master Chief John-117, one of a group of supersoldiers codenamed Spartans, and his artificial intelligence (AI) companion, Cortana. Other characters, such as Noble Six from Halo: Reach, are also introduced in the series. The term "Halo" refers to the Halo rings: large, habitable superweapons that were created to destroy the parasitic Flood. They are similar to the Orbitals in Iain M. Banks' Culture novels, and to author Larry Niven's Ringworld concept.