At the end of the Boss fight with Dominus Ghaul, he becomes a giant form of liquid gold light; he then turns to the Traveler as blue lines start to form and crack under the shield the Red Legion used to trap it, and that same shield is broken by the traveler.
Ghaul then says "You do see me..." as the traveler explodes and a wave of its blue light shoots out into the galaxy with itself as the center. This is the same blue energy that can be found on Io.
What is the traveler doing here? It's never really explained in the story, but it's visible from the tower for the rest of the game.