IMO it should be some ghostly-like ghoul with maybe the shade or voidwalker texture and some particles
also, i'm going to change the "shadow realm" a bit
the shadow realm wont be so much dark magic in general, but it's limited to demonic creatures and otherwordly things (ex, the Ulvons)
demonologists of course worship this place, but other dark magic users in general don't "worship" it as much.
the "light realm" will be similiar to "the light" in which paladins and priests/clerics follow in Warcraft