There is a thing that is not all clear for me, but I'm not sure how much could been seen a spoiler: http://www.funtasiadaily.com/2016/09/filly-funtasia-promotional-synopses.html
In one slide, is told that Wranglum was imprisoned in a dark crystal and hidden in a secret cave, and later one character help him to set free but at same time, he become prisoner of the Dark Mirror.
But in a second slide, it said that Wranglum was imprisoned in a mirror, and this mirror hidden in a secret cave.
I think the "canon" version is the one of TV show slides, but a possibility that I discussed with someone if is the mirror itself could be make with a Dark Crystal, or if Wranglum kept some Dark Crystal proprieties when he become prisoner of the mirror.
What do you think? That would be interesting in terms of story arc, becouse a certain character is affected by those crystals...!