If you listen to xür at one point he says, "your traveller has a dark mirror" which could mean a number of things. Xür being an agent of the nine could have over heard some crazy stuff about I dark traveller called like I don't know the follower( I'm going to call it that) which is the creator of the darkness much like the traveller is the creator of the light the follower theory can be supported by the xür quote but he could be talking about the darkness itself. since there is so much speculation there is an infinite amount of possibilities.