I believe that we found one of the oldest races in the galaxy the perfect blend of energy/organism/mechanism. This creature/race came under attack by the darkness/enemies. When the Traveler realized it could not stand against this foe alone it fled to our galaxy and looked for the most powerful/best suited race to be its proxy army. (to fight the war it never could). Knowing it had time it built us up (the Golden Age). After the darkness had arrived it knew it had equipped us enough to win. But we didn't have the soldiers. So it sent out its ghosts to give Earth its true army. Near immortal engines of destruction. (guardians). But it did not plan for the hive to steal a piece of it, or the cabal to unleash their siege weapons so quickly. This made the traveler sacrifice part of itself (the rupture). Now it lies dormant trying its best to heal. I don't believe it is a parasite because after the final mission if you look at the traveler you can see its light returning. But you could be right.