Well I am not sure if those black masked Ascians are even important enough for the balance of things and we still dont know if Shadowhunter truly killed the Ascians or if he believes them death because he does not know about their body hopping, thus he only killed those they possessed.

Read some ideas: Maybe its truly us that loses their body for a short time (all these strange scenes with us in the dark) and Kan-E-Senna says that about our body. And this out of body experience happens to have a chat with the WoD thus we learn of the (maybe) coming catastrophe.

I am really curious how these scenes will all play out in the end. Because after all we need a living WoL since people can still use the character after finishing the story, so it should not be something longliving.