Hello,
from Wowhead comments
"ANOTHER UPDATE: Another player got their GM response:
GM suggested looking in the Vault. Now, the first thing that comes to mind is Vault of Lei Shen because that's how you get hidden appearance for subtlety.
I personally think that this was actually a hint to Vault of the Wardens! Seems plausible, because it's intended to hold all sorts of dangerous creatures and tooltip text on (Item #139466) and (Item #139468) actually references a "prison intended for Thunderaan the Windlord", which may very well be in the vault. I'll try to check on this theory."
and i look quest guive by Wrathion and i found this quest=39579
Maybe GM talk about this Vault, i don't know.
And seems to catch with other GM respond Because this zone is very much phased
"Yet another update (10.09.2016): Another russian realm player got a response:
GM responded that "the final item is obtainable, but you'll have to wait a little because of a small phasing problem". Might be that we can't obtain the items yet because it's bugged somehow and is about to be fixed soon? GM also pointed out that some info may pop up on official website, so hotfix theory sounds good so far. This could also mean that the item will become obtainable once raids and world bosses become avaliable (Sep 21st). Still, other leads might be worth checking out
"