I would say Graendal's POVs might already contain confirmation of this - but others have already pointed this out in detail, so I'll just refer you to Theoryland.

Masema has had more of a role in the series than Slayer has, and his storyline ended in a rather one-note fashion. So I'm still not convinced. In fact, given that Slayer has more to do with Perrin's story than Rand's, leads me to believe that this is an enemy that Perrin will need to confront, and Slayer may have nothing to do with the Forsaken or the other characters.

And you're description of him dovetails exactly into why I find him and the theories about him so unconvincing: they all revolve around "His name is Slayer, which means he's deadly! And he has powers in T'A'R!"

By the way, we have absolutely no evidence that his powers are derived from the True Power. Firstly, it is possible for non-channelers to be Dreamers. Secondly, the Dark One can transmigrate souls, merge bodies/souls and affect the Pattern - all without using the True Power. We have no reason to believe the True Power is anything but a parallel force of the One Power that only channelers can access. Lastly, his powers in TAR seem to be less than that of a master Dreamer or Wise One - it looks like the only thing he does is enter it in the flesh. It is unknown if he can pull others into the dreamworld or etc.