Halo is a science fiction video game franchise created by Bungie. The main trilogy of games center on the experiences of the Master Chief, a cybernetically-enhanced human super-soldier, and his artificial intelligence (AI) companion, Cortana. The Master Chief aids future humanity in battling the Covenant, a theocratic alliance of alien races that store a parasitic organisms known as the Flood. In this science fiction setting, the term "Halo" refers to Halo megastructures: large, habitable ringed structures, similar to Larry Niven's Ringworld. The trilogy of video games have been praised as being among the best first-person shooters on a video game console, and are considered the Microsoft Xbox's "killer app". This has led to the term "Halo killer" being used to describe console games that aspire, or are considered, to be better than Halo.