The Traveler

The Traveler is a mysterious sphere floating above Earth. Massive in scale, it is unknown whether the huge celestial object is simply a derelict spacecraft or a sentient being in and of itself. At first glance one might mistake the Traveler for Luna herself, but this is no moon. Hanging motionless in the skies above the last human city, it protects the humans from the continuous onslaught of various alien races bent on humanity's eradication, and gifts the Guardians with ancient and mysterious powers.

History
At the end of the terrible wars that terminated the Golden Age, the Traveler, in its sacrificial last gift to humanity, came to rest over a small area of the now wasted human planet. As Humanity struggled to rebuild itself from these terrible events, the now dead object gifted its protection and powers to the re-growing humans. Although the times after the war were hard, with thousands dying to famine and disease, the remnants of lost battles, Humanity persevered. And with the help of this celestial guardian, the Humans eventually began to re-explore their terrestrial and solar domain.

The remnants of Humanity built a city beneath the resting place of the Traveler, where they were safe from the continuous onslaught of various alien races. This city rests in the shade of the great sphere, built under its protective aura as a last bastion of humanity. The Guardians are destined to protect this city and its inhabitants, and as such are gifted with some of the magnificent powers of the Traveler. It is not known if the Traveler is a sentient being in and of itself, or simply a derelict ship, but its importance to the survival of Humanity, cannot be understated.

Trivia
The Traveler first appeared in Bungie's previous game  Halo 3: ODST , where a hidden poster made it appear innocuously like the Moon.