One of the Sages is hidden away in the Skull Woods of northwest Lorule. Use the warp fissure from Rosso's House to get there.
Reach the Dungeon
The Skull Wood dungeon is in northwest Lorule. To get there, go to Rosso's House in Hyrule and go inside. Use the warp fissure in his house. Then in Lorule, merge with the wall to get past the bars, then go south until you can't go any farther south. From there, go west to where the Fortune Teller's Shop is in Hyrule, then go north. You will be in the Skull Woods. There is an entrance to the woods up the path.
Skull Woods B1
Be careful of the Wallmaster in this room. Watch for the shadow on the floor. Use the west exit.
Equip your lamp and go all the way to the south end of this narrow room. Step on the switch there, then go up and use the southern east exit.
In the next room, carefully make your way along the narrow paths, avoiding the Gibdos (mummies), the fire bars, and the Wallmaster. Go south and use the south exit.
Outside, go all the way east and then go north. Fall into the hole.
Back inside, go south and stand on a moving platform. When the Wallmaster's shadow appears, merge with the wall until the Wallmaster falls. Pop back out and stand on a moving platform to reach the ledge to the north. Open the chest to get a key. Go down the stairs to the east and go through the northeast door.
In the dark room, equip your lamp. A wallmaster will fall, so carefully go to the torch to the east and light it, then make your way north and then east, and finally south to a locked door. Open it, then go south through the exit.
Go north and fall into the hole. Open the chest to get a silver rupee. Go south and fall off of the ledge, then open the chest to get the compass. Go out through the southeast exit.
You will be back in the dark maze room. Equip your lamp and make your way southeast, and go again through the southeast exit like you did before. Go outside.
Go north, but this time don't fall into the hole. Go west and cut the bushes to find another hole. Fall into it.
Go south and then use the west exit. In the next room, go north and use a bomb or the hammer on the cracked tiles, or just let the wallmaster smash into it. Fall down below and go south, then step on the switch. Now go west, then north. Break the cracked tile here and fall down below. Open the chest to get a key. Then step on the switch to create a walkway to the right. Walk across it and go to the southeast corner of the room, then go west, then unlock the southwest door.
Go east and pull the switch to reveal a door in the north wall. Go into it.
Skull Woods B2
Merge with the east wall and go south to reach a ledge. Pop out and kill the moldorm, being careful to avoid the wallmaster. Push the two statues on to the two leftmost switches. Go up the west stairs and merge with the wall, then go east to where a switch is. Pop out and pull the switch. It creates a path downstairs. Now go out through the east door.
Go south and step on the switch. When the platform comes over to you, merge with it. When the platform goes to the walkway to the east, pop out there. Go north and stand under the switch, and wait for the wallmaster to fall onto it, activating the moving platform. Merge with that platform to reach the north ledge. Step on the switch here to create a shortcut up above, then go through the north door.
The door closes behind you, and you have to kill all of the monsters in the room. Go to the northwest corner and wait under the raised platform to make the wallmaster kill the gibdos, then do the same thing in the northeast corner. Then go step on the switch to raise the cages that contain the other monsters. Try to quickly get rid of a moldorm so you can hide in its cage if you need to. Get rid of the gibdos from the cage. You don't have to kill the wallmaster to end the battle. Afterward, a green portal appears. Step into it to go outside and save, then come back here. Go through the north door.
In this room, quickly go west up the stairs and stand on the moving platform. When you can, go onto the south ledge and go west, then run into the cage there. Let it take you back to the chest in the north middle of the room. Open it to get a key. Get back into the cage and then go onto the south ledge when you can. Go up the stairs and unlock the south door.
In this room, equip your lamp, then go all the way to the west side of the room and light the torch to make gibdos appear. Kill them to unlock the northwest door. Go through.
In the next room, go north and use the stairs in the north wall.
Skull Woods B1, part 2
Merge with the wall and go south to go through some bars. There is a wallmaster in this room. Merge with the northeast wall and pick up the eyeball. Throw it west so it lands on the ledge there. Merge with the wall again and go to the eyeball. Stand in front of the eye statue and press A to throw the eyeball into it. A platform appears to the south, and a piece of wall will move away from the big chest, but there is still another piece of wall around it. Go south along the platform, then go west and step onto a moving platform. You can merge with the north wall and go west to get onto the west ledge. Go south from there, then east. Merge with the south wall and go around in order to go farther south. It will seem like you are trapped, but there are actually bars in the south wall near the other eyeball, so merge with the wall to go through the south bars. Go through the south door.
Outside, go west and open the chest to get a purple rupee. Now go north and then east. Ignore the hole here. You will see the painting of Seres to the right. Go south from the painting and cut the bushes, then fall into the hole.
Merge with the wall, then go north, then pop out and go west along the stone wall. Go south along the walkway, then west, and open the big chest to get a chunk of Master Ore. Now go back to the southeast corner of the room. Merge with the south wall and go west to reach the eyeball. Pick it up and fall down to the lower level. Go east and throw the eyeball across the gap to the north ledge. Carry it west, hiding under the walkway if necessary. Go to the moving platforms and get onto one. When the wallmaster appears, throw the eyeball onto the nearest moving platform, then merge with the wall. Try to get the eyeball onto one of the platforms that is going east. This will make it easier to get the eyeball to the right side. Once you are on the right side after the moving platforms, go north and stand in front of the empty eyeball statue, then press A to throw the eyeball in. Open the big chest to get the big key. Go northwest and go downstairs.
Go through the south door, then go east and use the big key to open the door.
Knucklemaster will float around at first, then it will glow and charge up energy. When it charges up, merge with the wall to make it punch the wall. It will be stunned and will flip over, revealing its eye. Hit the eye until it flips back over. Do this until the eye changes color. At this point, sometimes Knucklemaster will try to fall onto you. When that happens, merge with the wall until it stops falling, but be careful not to run out of magic energy. Wait for it to charge up, then merge with the wall to make it hit the wall and become stunned. Hit its eye while it is stunned. Keep doing that until it dies.
Pick up the heart container, then go through the door that opened to the north. Touch the painting, then after you talk to Seres, step into the light.