When a human uses up all there energy (calories and stuff) they will eventually go into a coma/death trance that reduces to only the bear nessesities (the simple bear..nevermind) like the heart, brain, ect. This is essentially what the traveler is, it is only able to protect what is right next to it (the city), and even that only barely. The Darkness came, and the Traveler exerted all of its energy to protect us. Now the Darkness is returning, and there is no safety net this time.