From Destinypedia, the Destiny wiki
The Traveler is a mysterious spherical paracausal entity associated with the Light. Massive in scale, it hangs motionless in the skies above the Last City built in its shadow. Once responsible for terraforming the worlds of the Sol System, it was rendered largely inactive during the Collapse, when its ancient enemy arrived and launched a devastating assault on human civilization.
The Traveler is an ancient and immensely powerful machine entity, the source of Ghosts and the paracausal powers of the Guardians. It is a source of Light, though not the only source in the universe. Its outer shell is made of lead, neutronium and "electroweak matter", the latter based on a fundamental force that hasn't existed since the dawn of time.
The interior workings of the Traveler remain a mystery. The Ghost Balthazar claimed that Ghosts originate within the Womb, a place inside the Traveler described as a "cosmos inside a bottle" or an "infinite space enclosed within a celestial egg", where trillions of star-like souls orbit each other in a complex weave. Another Ghost named Peach was dismissive of this description, contending that neither she nor Balthazar can remember the time within the Traveler.Fenchurch Everis claims to have been inside the Traveler, but no one has been able to confirm the veracity of his story.
Origins and Purpose
The origins and true nature of the Traveler are unknown. Some information can be gleaned from the poem known as Dreams of Alpha Lupi; based on this work, it appears that the Traveler chose Earth and humanity for a reason; it detected a "face" emerging from the Sun, later hearing a roar that contained something that it described as "a lucid melody". The Traveler felt joy, as it felt "the first hope in ages transform [it]," and it resolved to become strong enough to fight the Darkness.
The Traveler seems to be motivated by pure altruism; it has never wished to be worshipped as a god, nor has it made its presence known any more than necessary to cultivate a civilization. In the past, whenever its work was finished, it would simply leave without fanfare. It would seem then that its defense of humanity was an extraordinary act under dire circumstances.
Saint-14 suggested that the Traveler had previously visited other species before finding humanity. The Fallen civilization in particular once hosted the Traveler, which they named the Great Machine. However, it left their system around the time of a catastrophic event only known as the Whirlwind. Other than the likely involvement of the Hive, little is known about this event, though it is thought to be similar to humanity's Collapse. After the destruction of their homeworld, the Fallen went in search of the Great Machine, leading them to the Sol System. All of their previous conflicts with humanity have been attempts to claim the Traveler for themselves, in an effort to prevent the destruction of their race
Other species the Traveler has also uplifted include the Ammonites and the Harmony. The Cabal are aware of the Traveler's existence and nature, with some indications that they knew of it from other species. Ghaul in particular wants to receive the same power as the Guardians, believing himself to be more worthy. It is later described in the lorebook Constellations that the Traveler felt more afraid of Ghaul, the Red Legion, and the cage device than its ancient arch-enemy itself, the Darkness. 
The Hive uniquely hold the Traveler in special contempt, as it embodies the antithesis of their culture and society. One of the objectives of their crusade is to hunt down and destroy the Traveler once and for all.
The Ancient Past
The origins of the Traveler are unknown. It is known to have existed for billions of years, during which time it has visited countless intelligent species, aiding their civilizations by paracausally terraforming inhospitable planets or realigning unstable orbits, among other things. It is known to have visited the Lubraeans, the Harmony, and the Ammonite billions of years before the present day.
The Golden Age
The Traveler first came to human attention when it appeared around Jupiter and terraformed two of its moons. Initially dubbed "Moon X", it quickly became the center of international attention. After finishing its work on Jupiter, it vanished for fourteen months, reappeared to terraform Mercury, vanished again for seven months, and then appeared again to terraform Venus. Questions about the identity of Moon X abounded, and cults worshiping it emerged, some of which fell into conflict. Other cults held demonstrations calling for the unification of Earth's governments.
When Moon X was detected approaching Mars, a mission to intercept it was hastily organized. Initially known as Project Catamaran, the effort was renamed Ares One on the day of launch. Four individuals were chosen to be apart of this historic mission: Jacob Hardy, the pilot and mission commander; Evie Calumet, a theoretical physicist who first discovered Moon X; Qiao, the navigator; and M. Mihaylova, a artificial intelligence specialist. While the team played a game of basketball and brainstormed ideas for the mission, Evie dubbed Moon X as the "Traveler" due it moving with a seeming purpose. By the time the ship landed on Mars, the Traveler had brought the first rain to the desert planet. It helped humanity expand across the solar system, teaching humans new technologies, helping them terraform planets and leading humanity into a Golden Age.
Some time later, the Traveler's enemy, the Darkness, appeared in the solar system, leading to the destruction of almost all human civilization in an event known as the Collapse. For the first time, the Traveler decided to stand and fight the Darkness, as it abandoned its work on Io and raced to rescue its "children", humanity. At the end of the 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. The Traveler laid dormant, apparently stripped of its senses and "flesh" by the Darkness, made blind, deaf and mute to the outside world.  As humanity struggled to rebuild itself from these events, the crippled object gifted its protection and powers to the recovering humans. Although the times after the war were difficult, humanity persevered and with the help of this celestial guardian, and the humans eventually began to re-explore their terrestrial and solar domain.
The Dark Age
In its last breath, the Traveler created the Ghosts, to seek out those who can wield its light as a weapon. The Ghosts revived fallen warriors as Lightbearers, with the intent of protecting humanity when the Traveler no longer could. Unfortunately, many Lightbearers would choose to become Warlords, subjugating the survivors of the Collapse throughout the Dark Age, where humanity faced the threat of not only the Warlords and the hardships of survival, but also the threat of alien races such as the Fallen. Lightbearers with integrity would band together to stand up to the Warlords in groups such as the Iron Lords or the Pilgrim Guard. Over time, the Warlords would be beaten back.
The City Age
The remnants of humanity built the City beneath the resting place of the Traveler, where they were safe from the continuous onslaught of the various alien races and the now-declining Warlords. This city rests in the shade of the great sphere, built under its protective aura as a last bastion of humanity. Over time, the people of the early City formed factions, dividing it and marking the beginning of the Faction Wars, which threatened to bring the City to its knees. It was during this time that a new generation of Lightbearers known as the Guardians emerged, taking a stand to end the fighting to focus on the outside alien threats. 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. Some believe that the Traveler chose Earth for a reason, and it is humanity's obligation to prove itself worthy of its light.
With the Darkness returning, humanity fears they will not be able to survive it this time. As a last ditch effort, the player is given the ultimate task of attempting to wake the Traveler before the Darkness arrives, in order to drive it back. The Guardians are able to help the Traveler slowly restore its light, first by stopping a Hive ritual in the Chamber of Night that is siphoning light from the Traveler, and then destroying the heart of the Black Garden.
Two years after the Taken War, the Cabal Empire's Red Legion launched a major invasion of Earth headed by the leader of the empire, Dominus Ghaul. A devastating surprise attack on the Last City was launched, trapping the Traveler in a cage to siphon off its Light while separating it from the Guardian forces. Though trapped, the Traveler sent visions to the now Lightless Guardians to seek out one of its fragments in the European Dead Zone, whereby they could regain their powers. Dominus Ghaul attempted and failed to win the Traveler's approval and make him into a Guardian, and simply took the Light by force. When he gloated in front of the Traveler, the Traveler suddenly broke its cage and destroyed Ghaul, as well as slowly healing itself by drawing fragments back into itself. With the invasion ended and the city retaken, the Traveler was declared no longer dead but living once more.
Despite having awoken, the Traveler has still remained hovering above the Last City and has not taken any direct action. However, Benedict 99-40 claims that the Traveler has been busy.
A year after the Traveler woke, Ikora Rey revealed that the Traveler had been speaking to her and several other Guardians through visions since its awakening. Ikora then instructs Guardians to follow visions of light. Eventually, the visions lead the Guardians to Io, where the remnant light there grants them new abilities.
An Old Foe Returns
As the Guardians continued to protect the Last City and the Solar System from various threats, ranging from a looming Lunar Hive Fortress, a resurgent Vex invasion, scheming Psion Flayers looking to subvert time, to the Red Legion superweapon, The Almighty, being turned into a kamikaze weapon directed towards the Last City, the Traveler's old enemy, the Darkness, slowly made its way towards the system. Though the Guardians soon gained a glimpse of the Darkness from their discovery of a Pyramid on Luna, the threat continued to ominously hang over them, especially as the Darkness made contact with The Guardian, addressing them not as their enemy nor their friend but as their "Salvation".
In time, following the Almighty's destruction, a massive fleet of Pyramid vessels descended upon the Sol System, confirming the Vanguard's fears that the Traveler's counterpart has returned. However, as the Guardians work in deciphering what the Darkness wants, the Traveler continues to hang motionless over the Last City, even as the Pyramids touch down on various worlds. Even Zavala noted that the Traveler appeared to do nothing to combat its ancient enemy. Nonetheless, the Guardians continue to work diligently on figuring out the purposes in the messages the Darkness is sending while protecting their holdouts in the system.
As the Pyramids approached and consumed Io, Mars, Mercury and Titan, the Traveler, in an event which saw all fireteams return to the Last City to witness it, gathered light in order to repair the damage which had been caused by the events of the Red War.
The Traveler later called out to the Crow, sending him dreams of flying over the European Dead Zone and other areas he had previously visited. When the Guardian went to investigate the locations featured in these dreams, they discovered paracausal feathers at each site. Later, the Crow and the Guardian went to a region of the EDZ featured in these dreams, and were led by a spectral hawk made of Light to a previously-undiscovered shard of the Traveler. On top of the shard lay an effigy of the Hand Cannon Hawkmoon, crafted out of the material of the Traveler's outer shell.
During the events of The Witch Queen, The Guardian discovers that the Traveler purposely chose Savathûn, the Witch Queen and her Lucent Brood for an unknown purpose. Shortly after Savathûn attempts a ritual to bring the Traveler into her throne world to either destroy it or save it from The Witness depending on whether she was lying or not. Regardless, Savathûn is defeated at her own game and is killed seemingly permanently and the spell is broken, causing the Traveler to return to the Last City's sky. Whether it truly wished to be brought to the throne world; wished for the Hive to supplant the Humans, Exo, and Awoken as its Guardians, and if it even purposely chose the Hive is still unknown.
The Traveler's emission of Light appears to undergo cyclical fluctuations. These fluctuations are tracked by the Tower and reported over loudspeaker in the Tower North. Traveler status announcements include:
List of appearances