Summary
To get the hidden artifact appearance for the Fangs of the Devourer, you must first give Marin Noggenfogger one these:
Vault Ticket http://www.wowhead.com/item=140774
After you turn the ticket and you want extra chances, give him one of these, as they only work after the first one.
Marin Noggenfogger's Lucky Coin http://www.wowhead.com/item=140774
After one of these has been delivered to him(Beware! the coins only work after you have given him a ticket that same day!), he will promptly allow you to take one item (And only one!) in his Vault.
Each day, the loot on the Vault changes, and no matter how many coins you give him, the loot available to get will always be the same until the daily quest reset on your server.
One of these items can be the Sparkling Set of Keys, which are here on this pillar marked with a small red X on the map:
and they look like this:
How to get the Ticket:
First you must get this research for the Class Hall:
It gives you a chance of getting a ticket upon a succesful follower mission completion. You can only get/turn one of these each day.
How to get the Coins:
Go to any dungeon or place in the broken isles with lots of humanoids and pick pocket them until the coin drops. Suramar City is a good place to farm for it, since there are a lot of humanoid mobs there and they are neutral if you have the disguise unlocked. Just be careful with mobs that break the disguise and pull you out of stealth!
The coins can be turned and looted multiple times every day, however they are Unique, so you can only have one of these in your bags at a time. It is advised that you turn yours in immediately after you get it. (Again, only after you turn in the Vault Ticket, else there'll be no loot.)
How to get the Keys:
Edit: Apparently, you have to turn in a ticket before you even try to turn in the coins, else no loot will show up at all with the coins.
After you turn in the coin the keys might not be there, that means no matter how many times you try in that day, the keys won't show up.
Just try turning in coins/tickets once a day and IF the keys show up, you can farm for the coins once more and if you turn them in on the same day, the keys are gonna be there again.
So if you get the keys, don't just bail. turn in more and more coins on the same day (they are unique, so turn one in exactly when you get it) and you'll have lots of keys(you get 1-3 keys each time you get them from the wall). I got around 15 keys in the day they showed up for me and it took me around 7-8 keys to get the appearance (I fucked up a lot on the scenario)
How to get the Appearance:
After you get the keys, Speak to Taoshi on the class hall and you'll start the Scenario. For tips on the scenario itself watch this video: (the scenario part starts at 2:13)