One year later...
Okay, I think I found more about that puzzling mirror. And if I'm right, then Deledrius' suspicions were correct.
First, the thing that I noticed in the trailer last time:
Last time, I was also wondering - if it's an illusion, could it have ties with the Guild of Illusionists ? But Illusionists operated in Katha, not on Ae'gura, so that was strange.
Except I'm beginning to wonder if the BCO was always intended to be on Ae'gura. Who knows, maybe originally it was supposed to be on Katha.
And lo and behold, a nice map of Katha that we never saw in HD until today's release. Bottom-left-hand corner: