The Scarlet Key
- For the Classic item, see
[The Scarlet Key]. For the quest, see 
[1-70] The Scarlet Key.
- The Scarlet Key
- Unique
The Scarlet Key is the master key to the most heavily-guarded areas claimed by the Scarlet Crusade. It is used to unlock access to the original, pre-Mists of Pandaria version of the Scarlet Monastery. It is not bound, and as such can be sold on Auction House.
While in possession of the key, head to the Grand Vestibule of the Scarlet Monastery exterior to spot an Old Keyring at [48.3, 56.3] or [83.9, 23.4] in Tirisfal Glades coordinates. It starts [1-70] The Scarlet Key, which unlocks the original instances.
Source
The key drops from the 
[Loot-Filled Pumpkin], the once-a-day award for completing the Headless Horseman encounter with a level 60+ character during Hallow's End.
Additional keys can be found in the Scarlet Monastery of Old in Doan's Strongbox behind Arcanist Doan in the Scarlet Monastery Library. The key can be obtained once per week per account.
Patch changes
Patch 10.1.7 (2023-09-05): Added.