One of the Seven Sages is in the Swamp Palace, south of the Vacant House. Use a Bomb Flower to complete the dungeon.
Use a Bomb Flower
There is a house just south of Thieves' Town in Lorule where you can borrow a Bomb Flower. Talk to the guy until he lets you pay 200 Rupees for a Bomb Flower. Go through the east exit of his house and check on the Bomb Flower. It will follow you around. Walk behind the big cracked rock and press the A button to put the bomb flower there. Then hit the Bomb Flower with your sword to make it explode.
Take another bomb flower, and this time walk with it to the area south of the Vacant House. Be careful not to let enemies hit it, because it will explode if they do. In the area south of the Vacant House, go south to the entrance of the Swamp Palace. Put the Bomb Flower in front of the big cracked rock south of the dungeon entrance and make it explode. This will cause the water to drain.
To complete the Swamp Palace, you need the Hookshot and Tornado Rod.
Swamp Palace 1F
Use the Hookshot on the switch across the path. Pull down on it to pull the switch and fill the pit with water. Swim across and go through the north door.
In the next room, there are some monsters that float around in shells. You can use the hookshot to pull them out of the shells and hit them with your sword.
From the central room, go through the west door, then go to the northwest part of the room and go down the stairs to the lower level.
Swamp Palace B1
Go to the east side of the room and go up the stairs. Use the hookshot diagonally to get the key that is on the raised walkway.
Stand on the raft and use the hookshot to pull yourself down to the wooden pillar below you. Now pull yourself to the left and open the chest to get a red rupee. After that, pull yourself to the right to open the chest there. It contains a potion ingredient.
Now go through the northeast door where water is coming out. Open the chest to get the compass.
Go back to the last room and go through the southwest door (it's hidden under a snake-head statue). In this room, go up the east stairs and throw a bomb at the crystal switch, but be careful not to let an arrow hit the bomb. Open the chest to get a golden rupee.
Go back to where you got the compass, then use the southeast exit. Use the Hookshot to go across the rushing water to the south. Then use it again to hit the crystal switch in the bottom middle of the room. The water will stop flowing. Go to the middle of the room and pick up the key.
Now go back to where you got the compass and unlock the door. In the next room, go northwest and hit the crystal switch to change the direction of the water. Go east and go into the flowing water. Use the hookshot to hit the crystal switch on the right. This makes some water flow to the west, filling a pit there. Go up the stairs north of the crystal switch and merge with the wall to go south. Walk into the flowing water. You'll end up in the new pool of water on the west side of the room. Use the hookshot diagonally to hit the crystal switch on the west side of the room. Another source of water will flow, filling the pool around the northern stairs. Go into the flowing water and then go up the stairs.
The door locks behind you and a giant yellow Bari appears. Hit it with your Hookshot if it becomes electrified, then hit it with your sword. After you hit it enough times, it breaks apart into a bunch of tiny yellow Biri. If they become electrified, hit them with the Hookshot. Otherwise, hit them with your sword.
When they are gone, a green portal appears in the center of the room. Use it to go outside and save the game.
Back in the green portal room, go southwest. Step on the switch to create a shortcut. Hit the bomb monster when it's above the cracked tile. It will explode and break the tile. Fall into the hole.
Go southeast and unlock the door. Use the hookshot on the left side of the round thing. Pull down and let go. This lowers the water level. Make sure the water is at the lowest level, then dive into the water north of you and go through the north door.
Swim north and dive when the spiky thing is rolling toward you. Go up and open the big chest to get the Blue Mail. It cuts damage in half! Awesome!
Now swim south to the previous room and swim south to the southwest door. Go through. Go to the center area and be careful to run out of the way of the Beamos's laser. Pull the left switch to open the door. (The switch on the right makes monsters appear.) Go through the northwest door.
Drop down to the right and step on the switch to open the northeast door. Now swim west to where you entered the room. Stand on the hookshot tile and use the hookshot on the stone thing on the wall. Merge with the wall when you touch it and go west around the room. Pop out on the southern ledge. Go east and use the hookshot on the right side of the round thing. Pull the right side down twice. This should raise the water level all the way. Get on the raft to the east and open the chest to get a key. Lower the water level once and go through the northeast door.
Go onto the platform with the flames surrounding the big chest. Use the Tornado Rod to blow away the flames. Open the chest to get the Big Key.
Swim through the door on the west side. Go northwest to the chest and open it to get a key.
Swim southeast to return to the central room. Now swim through the door on the east side. Swim to the northeast side and open the chest to get a potion ingredient. Now swim to the left side of the room and hit the bomb monster when it's on top of the cracked tile. When the tile breaks, fall into the hole there. Go southwest and unlock the door.
Raise the water level all the way and swim to the locked door in the southeast corner of the room. Go through.
Some shell monsters are there. Drop down to the lower level and use the hookshot to pull them out of their shells, then hit them twice with your sword. If you need to use a fairy fountain, go through the door that opens to the north. Otherwise, use the hookshot pole that was pushed by the water. Go up the stairs in the northeast corner.
Swim to the stairs on the left side of the room. Go up and hit the hookshot target on the wall to the left. Merge with the wall and go south along the wall. Pop out on the ledge with the two hookshot symbols. Lower the water level all the way. Get in the water and swim to the switch in the northeast corner of the room. Step on it to open the northwest door. Now use the water control to raise the water level. Go up the stairs to raise the water the rest of the way. Swim to the northwest door and go through.
Swim to the ledge in the northeast corner that has six skulls on it. Break the skulls and step on the switch that was hidden underneath. A platform will rise, allowing you to unlock the big door. Go unlock it.
Use the Hookshot to pull the eyeballs off of Arrghus. Hit the exposed eyeballs with your sword to kill them. Avoid the eyeballs that Arrghus shoots out at you. Keep doing this until the eyeballs are all gone. After that, Arrghus will bounce around the room. Don't let Arrghus land on you. Hit it with your sword to hurt it. Sometimes it shoots energy at the water, creating a circle on the water that shoots into the air after a moment. Don't stand on these circles. After you hit Arrghus enough times, it dies.
Take the Heart Container, then go through the hole that opens in the north wall. Go to the south side of the room and go up the stairs. Touch the painting of Oren. After meeting with Oren, step into the light.