One of the Seven Sages is sealed away in the Dark Palace of eastern Lorule. Reach it with a warp fissure south of Eastern Palace.
You need Bombs, the Hammer, and the Fire Rod to get through the Dark Palace.
Reach the Palace
To reach the Dark Palace, first go to the Eastern Palace in Hyrule and go south from it. There is an area with a warp fissure behind some rocks. Bomb the rocks and go through the warp fissure, then use the north exit from this area.
There are robot-like guards in this area that will throw you in jail if they see you. Avoid the red beams coming from their helmets. If you get caught, you can simply merge with the wall and walk past the bars to escape.
Go west past the jail. Merge with the wall to get past the guard there.
North of there, wait for the guards to be close to each other, then merge with the wall and go around to the top. Pop out when they're walking away from each other, and go north.
After going through the building, go west. To get some treasure, go south. Wait for the soldier to stare straight ahead, then merge with the wall just left of the red beams. Then go to the east side and wait for the soldier to stare straight ahead again, then pop out and go up the stairs to reach a chest that contains a red rupee.
Now go north. Wait for the soldier to be walking away and merge with the wall on the right. Go north and pop out when the soldiers are facing away from each other, then merge with the left wall and go around to the north and then east. You can pop out on the east side because there is no soldier to the east.
Now go east and south, then go up the stairs when the soldier is walking away. You can hide on the pillar that the soldier is walking around. There are rupees and a Heart Piece in this area, so get those if you want. When you are ready, go south from the first soldier, then down the southeast stairs. Get rid of the skulls in the way and go south.
Merge with the east wall to get past the soldier to the south. Pop out where the path goes east. Keep going east, then north from where a soldier is facing south. Merge with the north wall when the soldiers are walking away from each other. Then go to the north side of that wall. Wait for the northern soldier to be facing straight ahead and pop out of the wall, then merge with the wall north of you. Go past the northern soldier.
In the area to the north, get the rupees if you want, then merge with the north wall and go past the soldier.
Use the weather vane so you can get back here without needing to get past the guards again. Use a bomb to open the door to the Dark Palace.
Dark Palace 1F
Go to the northeast corner of the room and use a bomb on the cracked wall. You find a chest that contains a blue rupee.
Go back to the first room. Go to the northwest corner and set a bomb at the crystal switch. Run down to the square platform. You'll go north when the bomb activates the switch. Now use the Fire Rod near the boarded-up window. When the ball of fire from your Fire Rod hits the ground, it creates a column of fire that moves forward for a moment. The column of fire will hit the boarded up window and unlock the door. Go through.
There are some hidden things in this dungeon that disappear if you have your lantern equipped, so be sure to put your lantern away to see if there are any new things in the room.
Go to the northwest corner of this room and step on the switch. A key appears on a platform down below. This platform disappears if you have your lantern equipped, so put it away. Merge with the wall to reach the key. Go through the locked door to the north.
Go north past the rolling ball. To the east, there is a cracked tile. Break it with the hammer, then fall down below to get a key. Go through the door in the north wall to go back to where you were. Use the northwest exit.
Use the moving platform to go to the south side of the room. Plant a bomb, then pick it up and throw it at a crystal switch. Do the same with the other switch. When both switches are active, a chest appears. It contains the Compass.
You can go to the room to the south by putting a bomb on the moving platform so that it will explode when the platform goes to the bottom. It will break a cracked wall south of the platform. Go through to find a fairy fountain.
Exit through the east door. Merge with the wall and go north around to where there is a cracked tile in the floor. Use the hammer to break it.
Go back to where you came from and go east to the northeastern room. There are Goriyas in the lower part of the room. They mimic your movements. Be careful of the red one, because it spits a fireball when it sees you. The sword doesn't harm the red one, so use arrows or try another weapon. The northwest door will open when you defeat them.
Before leaving this room, go back up to the southern ledge and put a bomb on the crack in the middle of the south wall. Go through.
Go to the southeast corner to find a chest that contains a purple rupee. Now go back to the last room and wait for the moving platform to go to the bottom. There is a door in the wall below the moving platform. Go through. Open the chest on the left side to get a monster tail. Then use the hammer on the cracked tile in the northeast corner. Fall into it to get a potion ingredient.
Dark Palace B1
Down below, blow out the torch by swinging your sword at it, then unequip your lantern and drop down to the invisible paths below. Go north to get a key.
Go to the northwest room and hit the torch with your sword. Go to the northwest part of the maze and break the skull to find a switch. Step on it to reveal a chest. Open the chest to get a key.
Now use the key to reach the southwest room of B1. Get on the moving platform and throw a bomb at each of the three crystal switches to make the big chest appear. Open it to get the big key.
Now go to the southeast room of B1. Use your Fire Rod to hit the Goriyas above you. After they are gone, ride the moving platform to reach the green portal and go outside to save.
Back in the room on B1 with the green portal, use the west exit from the upper area. Merge with the wall and go to the west side of the room, where there is a crystal switch. Hit it.
Go back to the big central room of 1F, then use your key to unlock the door in the north wall and go upstairs.
Dark Palace 2F
Put a bomb next to the crystal switch, then merge with a wall that has a flame symbol on it. When the switch is activated, those walls flip. Pop out. If you go to the right side of the room, use the hammer on the cracked tile in the southeast and fall down. Open the chest to get a purple rupee. Now go back upstairs.
Go back to the east side of the room and use the northeast exit. The tiles in this room start falling, so quickly throw a bomb up to the crystal switch and run through the southern door.
In the big southern room of 2F, you can get into the secret room to the north by putting a bomb next to the southern crystal switch and then running up, using the northern crystal switch to get up to the upper level, then quickly merging with the north wall so that it flips with you on it. Pop out and open the big chest to get a chunk of Master Ore.
Now go back to the big southern room of 2F. Go to the north middle of the room and stand on the rectangle, then hit the crystal switch. Merge with the wall and go east to get to the ledge in the northeast part of the room. Use the fire rod to burn the boards on the window. Go south and use the fire rod again to burn boards off of another window. Drop down to the lower area and go to the west side. Use the fire rod to burn the boards off of the window there. Now go to the southeast corner of the room and merge with the wall to reach the chest in the east side of the room.
Now you can go back to B1 and go to the boss door. The lights are shining on the eyeballs, so the barrier opens, and you can unlock the boss door.
Light the torches in the northwest and southeast corners to start the battle.
Plant a bomb in front of Gemesaur King when it stops moving. After you have bombed it enough times, the mask comes off. Collect the rupees and start hitting Gemesaur King's face with your sword. After a while, it will put out the torches and start running around really quickly. Hurry and light the northwest and southeast torches to stop the Gemesaur King from running around. Hit its face with your sword again until it dies.
Take the heart container, then touch the painting of Gulley. After you meet him, step into the light.