After you retrieve the Water Dragon's Basin and help Scrapper reach the frog statue, you can enter the Fire Sanctuary.
Get a Key
Use the Beetle to bump into the green plant, known as Water Fruit, that is hanging over the lava in your way. Jump over the lava platform before it crumbles.
Knock down the Water Fruit to the left that is hanging near the lava spout. Ride on the lava platform, using your bow to kill the Bokoblins, and jump off at the end. Kill the blue Bokoblin up ahead to get rid of the bars and reach the blue chest, which contains a key.
Get a Second Key
Drop down below. An old Mogma named Guld will appear. He will mention the ancient treasure of this temple. The legend says "Ye who seek the entrance to the king's treasure, look for the two statues that face one another. Show your bravery and jump into the mouth of the sleeping statue. Do this, and the path will open before you."
Go through the locked door and watch the cutscene, which shows you the boss door.
Follow the straightforward path. Some Dark Keese will pop out and attack. Go up ahead and kill the two Bokoblins. Go through the door.
Go to the left to get 20 rupees, then go the other way. Jump down into the lava area. A Magmanos will pop up. Stand on the stone platform and the Magmanos will lift you up to the ledge.
Climb the vines to the right, kill the two Bokoblins, then climb to the right on the vines. Kill the Bokoblin at the other end. Hop down and save at the Bird Statue. Go through the door behind the Bird Statue.
A Dark Lizalfos will attack. Avoid its cursed breath, which will stop you from using items for a while. If it knocks you back with its arm guard, quickly do a Shield Bash to avoid its counterattack.
After you kill it, go through the west door. Bomb the Cursed Spumes or kill them with arrows, kill the two Bokoblins at the far side of the room, then make your way along the path and kill the Dark Keese that fly down near the vines.
Send the Beetle through the passage in the south wall. After it flies over a gap, there will be a few bombs. Pick one up and fly to the magma spout in the north wall that is partly blocked with rocks. Then use the Beetle to knock down a Water Fruit, and ride the lava platform to the far side of the room. Go through the non-locked door.
A Mogma is trapped above the lava. Ignore him for now and use the Gust Bellows to get rid of the magma chunks in the way. Open the blue chest to get a key.
Get the Mogma Mitts
Go to the previous room and go through the locked door. Cut the Water Fruits in your way. Go south from the wall of flames and open the chest beyond the Water Fruits to get 20 rupees.
Go up the southwest stairs to get an Evil Crystal from the blue chest. Go east from there and stand on the platform across from the frog, then do a sword thrust on the Water Fruit behind you. The fruit will get stuck on your sword and you can swing it at the frog statue to open the way.
Go north past where the flames were, and kill the Bokoblins and Dark Keese up ahead. Save, then go through the door.
Go down to the bottom, then get onto one of the stone platforms and let the Magmanos raise you up. Get a Water Fruit on your sword with a thrust. Then go back down to the bottom and run around to make a Magmanos come up. Hit the Magmanos with the Water Fruit to make it solidify, then repeatedly hit the Magmanos to make it crumble. Repeat with the second one.
After the lava disappears, go through the southwest door. Go to the bottom and use the whip to pull the red bar and free the Mogma. He will give you the Mogma Mitts. You can use them to burrow into special holes in the ground.
Get the Map
Go over to the holes near where you got the mitts. You can burrow into one of these holes. Follow the on-screen instructions to attack things in your path. Do this to break the rocks in the way. Press the targeting button to see above ground. Attack the bomb to make it roll into the debris in your way, then go around to the light shining down. Examine it to go back above ground.
Use the Gust Bellows to blow the magma chunks away. Save at the Bird Statue if you wish. Dig into the hole near the bars to burrow again. Go to the red side of the block and attack it to make the bars move out of the way. Go back to the light spot and emerge there. Then go to where the bars were. Go through the door at the top of the stairs.
Go to the right and kill the Bokoblin. Step on the switch to remove the bars, giving yourself a shortcut. Go back to the door that you came in through and use the bow to kill the three Bokoblins across the way.
Jump down to the lower level to find a Goddess Wall. Use the Clawshots to go back up, but be careful of the Bokoblin archer above the door that you came in through. Use the Clawshots to make your way across to the east door, and go through.
There is another trapped Mogma up ahead. He promises to tell you a secret about the room if you free him. Find the digging spot that you can burrow into. Go to the bottom of the area and flip the switch down there. Then flip the upper switch. If you do it the other way, the gates will block you and you will have to flip the switch again before you can flip the other.
Go back to the surface and do a thrust attack at the Water Fruit. Go up the stairs and throw the fruit at the frog statue. Pull the red bar down in the small room that you are now able to reach.
The Mogma plans to run away without telling you the secret, but then he recognizes your Mogma Mitts. He will bring up a blue chest and will tell you that there is a hidden room nearby. The chest contains the dungeon map.
Get a Third Key
The fake wall that the Mogma mentions is east of the blue chest. You can look at the map to see where the passage is, to see where to bomb the wall. After bombing the wall, burrow into the soil in this area to find the hidden route. However, there is a caterpillar. To kill it, hit the orange thing on its back when it stops moving around. You can hold the run button to move more quickly, but this uses stamina. You can also lure the caterpillar into dashing into the wall and getting stunned by making it see you at the end of a path, then backing away. Each time you hit the caterpillar's orange backside, it will run around more quickly for a while. After you hit it a few times, it will die. Emerge at the other spot of light in the left side of this area.
After you emerge, open the chest in this room to receive a key.
Get an Empty Bottle
Use the vines to go back to the previous area. Go south through the archway, then go west and through the door to return to the broken bridge.
Go across the broken bridge, then instead of going through the door at the end, turn left to go south through the hallway. At the end, go east across the non-broken bridge and through the door on that end.
Save at the Bird Statue if you wish, then go left into the small passage just north of the statue. Burrow into the hole, hit the switch to move the bars, then emerge again. Go north and follow the path. Step on the switch, then use a Water Fruit to kill the Magmanos down below. The lava will go away and you can go west to reach a blue chest, which contains an empty bottle.
Get a Piece of Heart
Go back the way you came, and run up the east wall to escape from the pit, then go north and follow the path. Go west through the door behind the Bird Statue, cross the bridge, then turn right at the door. Go through the northwest door. Go northwest to find a locked door next to a Bird Statue. Unlock it.
Follow the path to find some lava. Shoot the Cursed Spumes in the lava, and look west to see some Bokoblins that you can shoot. Use the Beetle to knock down the Water Fruit. Jump across. Kill the Red Chuchus up the stairs and go through the door. Kill the Bokoblins and Dark Keese. Go up the stairs, killing the Cursed Spume on the way. At the end of the path, burrow into the dirt. Switch the gate, then go toward the Mogma. The Mogma will switch the gate back, so go talk to him, then chase after him. Hold the run button to move more quickly. Once you intercept the Mogma, he will bring up a blue chest that contains a Piece of Heart.
Get the Mysterious Crystals
Shoot the Cursed Spumes and use the Beetle to hit the Water Fruit. Ride the platform to the other side. Step on the switch to lower the bars, then save at the Bird Statue. Burrow into the hole behind the Bird Statue. Go to the bomb and knock it into the rocks, then hold the run button to hurry back to the exit. Don't let yourself run out of stamina. Let go of the run button if you are close to running out, and use stamina fruit when possible. Emerge at the beam of light.
Go north, then go through the east door. Knock down a Water Fruit and ride it to the far end. Across from the Bird Statue, read the sign. Look down below until you see the "sleeping statue" that the old Mogma, Guld, talked about. Run off the edge to the left, and skydive to land in front of the statue with the closed eyes. A platform will appear below you as you get close. Use the Sailcloth to avoid getting hurt. Go up the stairs. Strum the harp where the butterflies are to reveal a Goddess Wall. Then go through the door at the top of the stairs.
Kill the two Dark Lizalfos. Go through the door at the top of the east wall. There is another Goddess Wall here that you can reveal by strumming the harp where the butterflies are flying. Go up the stairs. Use the Clawshots on the ivy, but before continuing up the stairs, look for the ivy in the gap in the northwest wall. Use the Clawshots to reach the gap in the wall, and open the chest to get a random rare treasure item. Then go back to the stairs and use the Clawshots on the ivy to proceed. Run to jump over the other gaps in the stairs. Open the door at the top.
There is a puzzle in here. Burrow into the hole and then step on the blue switches in the order of the number of wings on each statue. Press the targeting button to see the statues while you burrow. Hit the bomb to make it break the rocks in your way. When you activate all of the statues in the right order, you will have to kill the caterpillar to escape. Once you do that, emerge from the hole and open the chest to get the Mysterious Crystals.
Reach the Dungeon Boss
After you get the Mysterious Crystals, go through the west door. Step on the switch to remove the bars. There is a fairy in a jar here. Save at the Bird Statue and use the Mysterious Crystals on the locked door.
Ghirahim is here. In the first phase of battle, don't hit the daggers with your sword. Point your sword to lure Ghirahim's hand to one side until it turns red, so you can quickly slash from the other side of his hand (a side not blocked by a dagger). If he catches your sword, slash upward away from his hand to break it free. If the daggers fly at you from a distance, you can use a Shield Bash to deflect them. You will need to stay close to Ghirahim to hit him, however.
When he makes four daggers appear, use the same strategy, but you will have to be more careful about slashing in a direction that isn't blocked by his hand or a dagger.
Keep attacking him this way until he takes out two swords. When he makes daggers appear, stay back and wait for them to fly toward you, and slash in the appropriate direction to get rid of them. If they appear in a circle around you, do a horizontal spin attack. If they appear in an arc above you, do a vertical spin attack.
If he leaps into the air, run around and wait for him to land and get his swords stuck, then run back and try to hit him repeatedly before he gets up.
When he holds the swords in front of him, hit in the vulnerable direction, then quickly hit him multiple times until he jumps back. If you do it incorrectly, or take too long, quickly do a Shield Bash to avoid his attack.
Get Din's Flame
After you defeat Ghirahim, get the Heart Container, then go through the glowing door. Hit the crest with a Skyward Strike.
After your sword is transformed, Fi says to go to the gate in the Sealed Grounds.
Before you do so, you can go south into the caves, and dig in the soft soil to burrow through a tunnel that takes you to the cave on the east that you can see on the map. In the cave, you will find fairies and a Piece of Heart.
Piece of Heart from Gorko
You can find Gorko the Goron on the path between the Sealed Temple and the Faron Woods. If you talk to him, he asks you to show him different designs drawn on the nearby Goddess Wall. If there are no butterflies near him, you can make the Goddess Wall reappear by saving and quitting, then resuming the game, or by leaving the area and coming back (e.g. by going back to the Sky and then returning to Gorko). After you draw the first shape that he has asked for, he will give you a Piece of Heart. For more information, read the Goddess Wall Guide.
When you are ready to move on, go back to the sky and fly to the green column, and land at the Sealed Grounds statue.