Ye this one has got me stumped as they can't look demonic as demons represent chaos we cant make them undead themed as that represent death. So to make something that is the other side of light would be pure dark whats not that interesting looking at a black screen.
But from the light demon we see in legion it makes me think that light and shadow doesnt actually have a form of its own but rather alters others depending on there original alignment as we can see example from death wing to the old gods and the chained elements plus the emerald nightmare as there all infused with void lord powers but have unique looks. This could be a good thing going forward as it means that they can make many different void lord representing bad guys but with unique themes. But one thing is for sure is they all share tentacles!!!! in common