"Everything changed with the arrival of the Traveler. Long ago, it breathed new life into our system and began our Golden Age. But it did not last. Something hit us. Knocked us down. Tried to stamp us out. No one knows exactly what, but we do know this: Very few of us survived, and those who did owe their lives to the Traveler."
The Traveler is a massive orb of unknown origin or function that hovers above the last settlement of humanity on Earth, known as the City. No one knows how or why it arrived, but its presence gave humans abilities which sparked their expansion into the solar system during a period of time known as the Golden Age. After the Collapse, the Traveler sacrificed itself to save humanity, and now serves as protector of the last bastion of human civilization by providing a defensive shield around the City and imbuing the Guardians with abilities with whatever power it has left.