Just came up with a new scenario. Perhaps mankind's golden age of space travel turned into hostile expansion into neighboring systems. Humanity starts attacking other species one by one taking planets, colonies and resources. The traveler is just one part of an automated peacekeeping system left a previous "caretaker" race (which would fit in nicely with some of Bungie's themes). The traveler takes notice of humanity's actions and starts fixing the the problem. The other species unite behind the traveler, but humanity puts up a hell of a fight and eventually gets pushed back to earth. Both sides' resources and numbers are almost completely exhausted. The Traveler, crippled and nearing its own death due to the prolonged battle, lies to humanity. It let's humanity believe that it still has the capacity to wipe them out so it offers them one last chance to save themselves. Humanity must cease hostilities and must never again venture into the solar system. Humanity resists and the traveler uses the last of its power to wipe out a majority of the population. Humanity begrudgingly accepts the offer and the traveler goes comatose. Generation are born and die raised on the lie that the traveler saved humanity from space bogey men and eventually it becomes the truth. Humanity wants to return to the "golden age" so they find a way to tap into what's left of the comatose traveler's power and uses it on their righteous quest to reclaim their former glory and repel the invaders.