Crimson Desert is one of those games that does a very poor job at explaining how things work. Especially, where to use certain items in the game like an Engraved Key aka the Mystical Key which we don’t understand where to use.
The Mystical Key unlocked a door; once you have the key open your inventory and select the key to examine it. Examining it will put your character in first person mode with the key in focus. Hold down the LB/L1 to examine the key and unlock the next portion of the quest.
How to Find the Room the Key Opens in the Mystical Key Quest
The mystical key is used to unlock a door in the Hernand Castle. However, you can’t get into the castle without proper attire. So of course, we need to get the right attire first by completing an errand. You’ll need to complete the Actions Speaker Louder than Words quest where you must help a couple clean their chimney,
The couple is at Murkoot Ranch. Head over there and wait for the cutscene to trigger when you approach them. The husband falls off the roof while cleaning the chimney and the job falls upon you.
Grab the broom placed outside the entrance of their house and climb up on the roof. You can climb up from the front, but I suggest going through the back so you can pick up some loot.
Now press down on the left D-pad to open the wheel and select the broom to equip it. Stand on top of the chimney, press L1/LB to aim while standing on the chimney, then press R1/RB to clean it.
Once the chimney is clean you will be rewarded with the Hernandian Attire. Equip the attire and head to one of the gates of the Hernand Castle. The guards will allow you to enter the castle. You need to reach the second floor of the castle
As soon as you enter the main building/inner keep, turn right and go up the stairs to the second floor.
At the top of the stairs, turn right again into the next hallway/corridor. In this hallway, go to the first door on your left (it’s often described as immediately on the left after turning; avoid the guarded door farther down or other doors).
Approach the door with the Mystical Key/Engraved Key and it will automatically be used to unlock it (no manual prompt needed in most cases).
