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Start by heading east in this area. You'll need to chase down some Nocturns and defeat them using Zelda's phantom abilities.

After dealing with the Nocturns, look for two switches. Have Zelda stand on the switch located in the northeast corner. You'll then need to stand on the other switch, which is near the door you came through. This will open the door, so head on through.

In the large room ahead, proceed north. Be very careful of the pitfalls scattered around this area – don't fall in! Navigate around the corner into a more secluded section.

You'll encounter a few Rats here; take them out. Then, make your way to the northwest corner of this room. You'll find a chest there; open it to get a piece of treasure.

Now, head to the southeast corner of this floor. You'll find a red door. When you try to open it, it will ask you to draw a specific symbol that can be found somewhere in this room. You can draw anything to exit this prompt for now.

The symbol you need is actually related to the Boss Key. Head back to the large room with all the pitfalls. You'll notice some weird tiles spread out in lines on the floor. Follow these lines, and you should see that they form a symbol. It's actually the letter "Z"!

Return to the red door and draw a "Z" on the screen. The door will open!

Inside, you'll find that the Boss Key is electrified. This means only Zelda can pick it up and carry it. As soon as she grabs it, a bunch of scary Key Masters will appear.

Your next task is to protect Zelda from these Key Masters as you both make your way to the Boss Door. Thankfully, the floor will be lit up during this sequence, making it easier to spot the pitfalls and avoid them.

To have Zelda throw the key onto the door, you'll need to trace a path for her directly into the door. You'll see a crosshair appear when you're aiming correctly. Once the key is placed, the Key Masters will disappear. Go upstairs to proceed.