Hit the crystal switch in front of you to make the blue blocks go down. Then head down the stairway that was previously blocked by the blue blocks.
Kill the stalfos. The pink-skulled one will throw a bone at you when you swing your sword at it, so take caution. Try to kill the moldorm (wormy creature) while it's still inside the orange blocks. Then hit the crystal switch to lower the blocks, and grab the key. Be sure to hit the crystal switch so the orange blocks go up again, or you will be blocked upstairs by the blue blocks. Go upstairs.
Hit the crystal switch below you to lower the orange blocks and raise the blue ones, and go north. If you touch the tile, Sahasrahla tells you that you can use the Magic Mirror inside any dungeon to return to the entrance! Very nice!
Continue north to where a moldorm lurks and two red, horned monsters pace inside blocked off areas. Kill the beasts, then open the chest above for the map. Unlock the northwest door and go downstairs.
This is a room in which the tiles fly up and attack you, and this time you aren't safely inside the doorframe. You'll have to break the tiles with your sword before they hit you. Your best bet is to hold down B so that you hold your sword in front of you. When a tile comes near, you automatically tap it with your sword, breaking it. For the two top corner tiles you may have to turn to the side to safely break them, but otherwise this strategy should keep you safe. Hit the crystal switch and go through the door that is now open.
Kill the moldorm out in the open, then hit the crystal switch and try to block one of the moldorms in so you can kill them one at a time. Once they're all gone, lower the orange blocks and go through the south door.
Kill the pink-skulled stalfos carefully (don't be tempted to use the pots; they hide magic jars that you will need in a few moments). When they are gone, light the four torches here with your lamp to reveal a chest containing the Big Key. Get the key, then lift the pots for magic jars (one is a big-sized jar that fills your meter completely). Go to the last room.
Hit the crystal switch in here, then proceed to the next room, where you should hit this crystal switch to raise the blue blocks. Go upstairs.
Go south and east to an unblocked stairway. Go upstairs.
Avoiding the holes, kill the hardhat beetles carefully. When you hit them, you get knocked back a bit, so try not to get knocked into a hole. Don't hit the crystal switch! Just go through the door.
When you step on the star here, the holes in the ground will change position. If you wait a moment, you can cause a hole to appear underneath the hardhat beetle below you! Kill the remaining hardhat beetles, then go up through the orange gap, open the big key door, and go through.
Kill the attacking hardhat beetles, then go east through the orange gap, killing an hardhat beetle on the way. Step on the star switch here to change the position of the holes in the floor, and go east, up the stairs.
Make your way to the center of the room, where you find the compass. If you touch the tile here, Sahasrahla tells you that you must find the Moon Pearl. Well, duh! Continue west, killing the fire-breathing monsters, and go up the stairs in the northwest wall.
Hit the star switch near the spinning flames to change the floor holes. Go into the center of the room killing hardhat beetles, then head to the north area of the room in the center. There should be a hole here; drop in from the top edge to find yourself standing before the big chest! Avoid the circling fireball and open it to get that sacred orb, the Moon Pearl!
Now head south over the star switches to get past. Head up the northwest stairs again.
Head to the eastern side of the room, where many pots are sitting. Each contains a heart, so lift some pots if you're low on health (but don't waste any hearts; you will probably need them for this next, irritating boss). Then, when you're ready for the boss battle, head up the stairs.
-- Moldorm --
Head to the bottom of this room, where there is a ledge into Moldorm's pit. Unlike its smaller kin, this creature's only vulnerable spot is the red base of its tail. Hitting any other part of its body will knock you back. To complicate things, the platform on which Moldorm squirms is surrounded by a gaping hole on all sides; if you get knocked off the edge, you will have to come back upstairs and start the battle over again! As I said, it's quite irritating. Once you hit Moldorm five times, it will start to move even faster around the platform, making it even harder to hit its blasted tail. One more hit will finish it off. Be patient, and do your best. You may want to stay to the left side of the rectangular hole in the platform; it's easier to avoid Moldorm there.
Once you've destroyed Moldorm for good, collect the heart container and stand in the center of the platform, where the third and final pendant will drop into your hand. Your health and magic is restored and you are returned to the outside of the dungeon!