Unless the fix for Withered J'im came in before the weekly reset, if you have been downing world bosses every week, you should be able to get the Unleashed Monstrosities achievement today, with the Soultakers popping up in Stormheim (note that if you're a Death Knight, you can also get the Frost DK hidden appearance off them. I believe it's a guaranteed drop. I ran the bosses as Frost just in case that mattered. You need to loot it off their bodies.)
Now, if you, like I did, sprinted back to your Order Hall, eager to try out the new artifact appearance that you just theoretically unlocked thanks to that achievement, I've got bad news for you.
These "third tier" artifact appearances are all locked behind the Balance of Power quest chain (which can't be completed yet, as it apparently requires you to do some Nighthold as well.)
Personally I'm a little upset about this: the artifact appearances are pretty limited already, and putting not just one appearance/tint but a whole row of four behind this chain is pretty frustrating. The worst part of it is definitely that there's nothing in the game's UI to warn you about this. On the artifact altar thingie (whatever your class uses) it simply says that you unlock that square by getting this achievement.
Again, it wouldn't be such a big deal if there were more looks you could unlock in simpler ways.
I'd still recommend getting the achievement - world bosses drop very nice gear and you might get that achievement eventually. Still, regardless of whether Blizzard posted about this before (as some on the forums say they did,) I think making it clear within the game would have been smart.