If you have the whole disk you should be able to open the door in the NE part of the map. If I remember correctly you head south from the lava room and you come to a hallway where you can head east. You go east and a hallway branches off to the north that leads to the letter tile puzzle room. If you ignore that branch (I'm assuming you've already done that) and continue East the hallway will turn north and you'll come to a door. Open the door and you'll be at a staircase to head down and tangle with the Shade Dragon.
Drink up.
-Tipsy
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