To get the key to the secret room in the Queen's Maze, you need to find this specific Level 7 sea cave which is randomly spawn on the map, and get the key from the final chest in this sea cave.

Note from my experience:

1. It is a one-tile Lv 7 sea cave on the ocean (I have only found it once so if anyone has other experience feel free to comment and share).
UPDATE: You must clear every other sea caves in order for this Lv 7 sea cave to spawn! (Thanks to Azearon's & brazzer Jackob's comments!)

2. It is not on any big continent, so you don't need to check every tile on big continents that have towns and dungeons.

3. Useful classes to find this sea cave:

• Scholar: You need to get a lot of vision scrolls to open up the vision on the ocean. Scholar's passive skill works well. Additionally, teleport/portal scrolls are good for teleporting from Alluring Pools to nearest dock in order to open up a boat at a new ocean area.
  
• Pathfinder: High talent class for more successful boat movement rolls, passive skill for a chance to reveal sea cave nearby.
  
• Any other high talent class for ship movement

4. Useful items to have:

• Items that give bonus movement
  
• Items that give bonus find distance
  
• Vision scrolls
  
• If possible always upgrade to the biggest boat you can afford, because bigger ship offers more bonus movements
  
• teleport/portal scrolls to move from Alluring Pools to Docks when you want to start searching another ocean area

5. Although technically you can skip all the saving people main quests to find this cave, successfully saving people does extend the duration before the next person is going to die. So saving people will reduce the chaos speed, and give you more time to find the sea cave before the deadline for the final dungeon. I only managed to find the sea cave at like 2 rounds before the final dungeon deadline, so time is a concern before game over.

6. You will get a steam achievement for finding the secret room in the last dungeon. And you do not need to fight Johnsgrave, he will join your team and fight Hangman together instead.