Well I haven't got that orb myself yet, and honestly I have not read throught every single comment in this post, so my idea might has been exposed already.
Anyway, about that orb, it has something to do with the Dreadguards stuff, since it a) requires you to defeat a "dreadtooth" first, and afterwards you can summon a boss which comes right before a dreadguard boss.
So, you might try using it somehow on the Dreadguard Master themselves?
I mean, Asation has been upgraded to lvl 55. Darvannis has another Dreadguard Master to fight with.
The raids comming in further patches probably will have the remaining Dreadguard Masters as boss encounter.
That Dread Orb looks like some help against those Dreadguard Masters, like a shield or some damage buff?
Or maybe we need it in some future for the last of the Dreadguard Master encounter, which might be like a one one boss encounter with huge expectations?
Or, on the other hand, it is just another rare item doing some special effects with no other use, same as the Party Bowl or as they have in other MMORPG's.
Anyway, would be interesting to know what it really is for
If you have already completed S&V (and thus it won't be spoiled) read my above post (395). It's pretty much the same as what you said, but a bit more detailed. Especially considering it can be tried right now...