Waz sup 360 is right. Cortana soley saying that would make no sense. Gravemind is definately using her to make transmissions. "I know you, your past, your future" this is gravemind talking about how he knows of the human's Forerunner past, and he knows the way it will turn out again. "This is the way the world ends". I think Gravemind is going to have a very deep role in the plot. Possibly makes his way to earth somehow? But beside all this, the thing that realy interests me is how it seems the Covenant in the trailer have uncovered the arc, and have just activated it. Then followed by "this is the way the world ends" My idea of this is.... That this is actually the end of the game they are showing, but when u play the game, it will start totally differently to this. Possibly with Chief in the forerunner ship. Maybe Gravemind is showing an image of what "may" happen if the covenant succeed. Ah well who really knows?