Is it stated somewhere what the hell is this vortex-thing in the sky? It's in example in ghost world, icecrown, twilight highlands and dread wastes. Is it anywhere explained in lore?
Id say its just a skyeffect based on the controlling infuluence of the zone Lichking, twilight hammer, sha. If u were evil and had influence over the weather you'd want the sky to look a lil spooky adds ambience to the zone