The Revelry Hall Secret Key in Where Winds Meet looks important, but the game does very little to explain what it is for. It does not tie into the main Revelry Hall story step on its own, and there is no on-screen hint pointing you toward the lock it opens.
What the Revelry Hall Secret Key is for
The Revelry Hall Secret Key is used inside Kaifeng’s Revelry Hall itself. It opens a locked door on an upper floor that was previously inaccessible, leading into a hidden “mysterious face” room tied to Jianghu Legacy content.
You only gain access to the correct floor once you have progressed a specific Jianghu Legacy story. Before that point, the relevant level of the building is blocked off, so the key will sit in your inventory without a clear use.

Unlocking the correct floor in Revelry Hall
The door the key is used on is not available during your first visit for the Chapter Two main quest “Revelry Hall.” That main story step simply brings you into the hall and ends once you proceed inside, rewarding Commerce Coins, EXP, and Kaifeng exploration progress.
To reach the locked door, you first need to complete the Jianghu Legacy story Gambit Beneath the Shimmer. After finishing that legacy story, you become “able to access the fourth floor to enter the mysterious face room.” That fourth floor contains the door that finally accepts the Revelry Hall Secret Key.

How to reach the locked door in Revelry Hall (Kaifeng)
Once Gambit Beneath the Shimmer is complete and the upper level is open, you can physically reach the door that uses the key. The important detail is that the door is not on the very topmost level, but on the newly accessible floor just below it.
Step 1: Fast travel or walk to Kaifeng City and enter Revelry Hall through its main entrance, as you did during the Chapter Two story segment.
Step 2: From the main interior where the crowd gathers, locate the queue of fan girls/NPCs waiting in line. This queue serves as a useful landmark for the right staircase.
Step 3: Go up the nearby stairs from that fan girl queue. Keep climbing until you reach the level that was previously blocked off during earlier visits. This floor is now open after Gambit Beneath the Shimmer.

Step 4: On this newly accessible floor, turn right from the top of the staircase. Follow the corridor; you will find a locked door that prompts you to use the Revelry Hall Secret Key.
Step 5: Interact with the door to consume the key and open the way into the hidden “mysterious face” room on the fourth floor.

What to expect after opening the door
Using the key grants access to the mysterious face room on the fourth floor of Revelry Hall. This space is treated as a secret area related to Kaifeng’s Tales and Echoes and Jianghu Legacy progression rather than the early main story.
Inside, you can explore and claim the associated rewards, which are positioned as “secret” loot for players who push through the Gambit Beneath the Shimmer legacy story and remember to come back with the key. The exact contents can vary with your progression, but the important point is that the key’s only known use is to open this specific door; it is not a generic master key for the rest of Revelry Hall.
If you are stuck holding the Revelry Hall Secret Key with no idea where it goes, the checklist is simple: finish Gambit Beneath the Shimmer, return to Revelry Hall in Kaifeng, climb up past the fan girl queue to the now-unlocked upper floor, then turn right to find and open the fourth-floor locked door.