Halo 2
The Halo of the game's title is the name of a series of planets 'built' by a mysterious race called the Forerunners a long time before the game. The Halo's are the last defense against another alien race known as the Flood. The Flood want to use every living thing in the galaxy for food. The Halo's are actually giant super weapons, that when activated kill all life in the galaxy. This is the only way to stop the Flood, to starve them by killing all their food. The Covenant are not aware of this and are attempting to fire Halos as part of a false religious prophecy. In the first Halo game, Halo: Combat Evolved, the humans and Master Chief manage  to stop the Covenant from firing one of the Halos. They also destroy one of them in the process. Just before Halo 2 begins the covenant discover another Halo that can only be got through a portal found near Earth.