It was something to be sure. A black pulsating liquid metallic blob that vaguely resembled a heart and needed to die in order to help the traveler. Not a lot of in game clarification there. The deeper you dig into the story, the more you realize there isn't really one to find.

I think it was just something from the darkness that was stealing light from the traveler. So yeah, you're right. But restoring light to the traveler is pretty much the main reason why guardians are fighting, and being able to take out something so important so quickly is just confusing. And didn't feel very significant. "Think you can kill a [b]God[/b]"? Apparently we killed a God already.