The Scarlet Key

If you have successfully gotten yourself [item=7146], a keyring will appear at the top of the stairs as you head into The Grand Vestibule inside the outside world version of [zone=796] in Tirisfal Glades.

The first time you interact with the keyring, it will give you this quest The Scarlet Key, it's an immediate turn in at the same time. This will give you account-wide access to the old version of Scarlet Monastery.

To gain access to the old version of Scarlet Monastery, simply click on the Old Keyring to toggle The Scarlet Key on and off. This will give you a buff on your character called "The Scarlet Key - Retracing the steps of those who came before."

With the Scarlet Key buff active, the portals for Mists of Pandaria versions of Scarlet Halls and Scarlet Monastery will instead zone you into the old Scarlet Graveyard and Library, and the two doors that originally led to the Armory and Cathedral are once again open!

If you can't see the key chain and quest as soon as you walk up the steps in the entrance, you're in the wrong phase of Tirisfal. You need to talk to Zidormi and ask to see Tirisfal from before the Battle for Lordaeron. If you see the blight scar at Undercity, wrong phase. The Thomas Hallows Eve quest line also requires you to be in the "before the battle" phase too.

Are you 100% sure it you got it from a Candy Buckets at Inns? This is the only mention of it I can find. Everyone else just mentions Loot-Filled Pumpkin