After you rescue Erik from labor traders in the town of Freejia, you learn where the Diamond Mine is and go there.
Explore the Mine
To the north, there is a Flayzer. Kill it and go north. A Grundit pops out of the ground. Kill it and go east. Kill the two Flayzers, then go north and kill the Grundit. Go north and kill another Grundit, then go west from there. Kill the Flayzer to increase your HP. Go northwest and down the stairs.
Go east and kill the Flayzer, then go north and kill the Flayzer there. You can ignore the elevator door for now, because you don't have a key. Go northwest and kill the Flayzer who is attacking the laborer. Hit the ball and chain next to the laborer, then talk to him and he says that there are a total of eight laborers trapped there, including him. He leaves.
Go east, then north. Kill the Eye Stalker. It might close its eye and become invulnerable if you go close to it, so walk away to make it open its eye again. Kill it, then go west and north and kill the Flayzer. Go north and east. There is a closed fence with two Eye Stalkers in it. Kill the Flayzer south of the fenced area, then go northeast and kill the Grundit. The fence will open, so kill the two Eye Stalkers.
Go east from there until you see another laborer. Go south and east, then kill the Grundit, and go north to find two more Grundits. Kill them, then kill the Flayzer, break the ball and chain, and talk to the laborer.
Go west and kill the two Eye Stalkers. Then go east and south and kill the Eye Stalker down there. Go farther south and kill the Eye Stalker, then go east and kill the Grundit. Go north and open the chest to get an herb.
Go all the way south, then west, and kill the Grundit, then the Flayzer. Go north and kill the Grundit, then go north and up the stairs. Go northeast and kill the Grundit, and hit the switch. Then go west and south and kill the Grundit there, then hit the switch. Then go north, then west, and kill the Flayzer to increase your STR. Hit the switch. Finally go north and hit the switch up there. The stairs in the middle will open. Go south and go down those stairs.
Go east and kill the Flayzer, then go south down the stairs. Go west and kill the Flayzer, then go west and south. Go east and kill the Eye Stalker, then go north, then west, then down the stairs to the south. Go west from there and kill the Grundit, then go north and kill the Flayzer. Go east from there and kill the Grundit. Your DEF will increase, and you will get access to a door to the Dark Space.
Before you go into the Dark Space, go all the way west and stand near the wall. You will notice the wind blowing your hair. Use Psycho Dash on the wall above you to break it. Talk to the laborer to get Red Jewel #15, Red Jewel #16, and Red Jewel #17. Then go to the Dark Space and transform into Freedan. Leave the Dark Space and go west, south, and east, then go down the stairs to the south.
Go down the stairs, then go west and kill the Flayzer. Go west and north and kill the Flayzer and Eye Stalker. Go east and north, then west and kill the two Eye Stalkers and the Grundit. Go east and north, then go east and up the stairs. Go down the ramp and keep holding left to go up the ramp on the other side.
Go north and kill the Grundit. Go north and down the ramp, or go down the stairs east of the ramp. Kill the Grundit and Flayzer to the east, then go northeast and kill the Grundit. You can't reach the Grundit up above, so go east and down the stairs.
Go north and approach the Eye Stalkers to wake them up. When you kill the northwest one, it opens the fence up above. Go north and kill the two Eye Stalkers, then go north and kill the two Flayzers. When you kill the one on the right, it opens the fence to the north. Go north and kill the Grundits in the northwest and northeast corners, then kill the Eye Stalker to increase your HP and open the fence up above. Go north to find a door to the Dark Space. Tranform back into Will, then go outside and go up to the northwest part of the wall. Use Psycho Dash on the part of the wall that makes your hair blow in the wind. You will open a door in the north wall. Go through.
Up ahead, there is a new door to the Dark Space. Inside, you can go up to the statue of Freedan to receive the Dark Friar power. This allows you to kill enemies that your sword can't reach. It works like the Psycho Dash, but instead of dashing forward, you shoot a beam of energy from the sword. Only Freedan can use the Dark Friar power.
Leave the Dark Space. Go south to go to the next room, then go south through the long room and go south to exit. You should now be back in the room with the Grundit north of the ramp that you couldn't reach before. Use Dark Friar to kill it. Just swing the sword (make sure you are in Freedan form) and hold the button until you start glowing rapidly, then let go to shoot the energy beam. This removes the wall between the two ramps. Go west and up the stairs, then go down the ramp. Be sure to avoid the skull that is between the two ramps. On the other side, go east and down the stairs.
You find a chained laborer. Cut the ball and chain, then talk to the laborer. He gives you the elevator key. Go south to exit this room.
Go down the ramp. Go to the northwest corner of the room either by using the ramp or going up the stairs. Go south from there to reach another ramp. Go down that ramp, then make your way south. Go to the southeast corner of the room and go up the stairs, then go into the door to go down to the lower level.
Go north up the stairs, then go east and make your way around the spiral until you reach the door in the northwest corner of the room. Go through.
Go west and fall down the ledge, then go south to reach the locked door. Equip the elevator key and use it on the door. Go down the steps.
Follow the path south, then go onto the wooden platform and walk west. In the next room, stand on the small wooden platform. It automatically moves. When it stops, walk down onto the wooden platform and go west. In the next area, go all the way south to reach the next room.
There are three holes in the ground here. You can visit them in any order. The one on the left goes to the Morgue. Go in and kill all of the Grundits (use the map to see where the monsters are) to increase your STR. In the southeast corner of the room, there is a sparkle on the ground. Walk onto it to find Mine Key A.
Go back upstairs and go into the door on the east side. There is a maze here. Go east to find the entrance to the maze. There are no dead ends. Go through the maze, killing Eye Stalkers and Flayzers as you go along. You may need to use the Dark Friar technique to kill the Eye Stalker near the chained laborer since the fence is closed there. Go to the chained laborer and kill the final Flayzer to increase your HP. Then cut the ball and chain and talk to the laborer. He gives you the Mine Key B.
Go back through the maze and go upstairs. Check on the locked door in the middle and you will be asked if you want to use both keys. Say yes.
Go downstairs to find the three child laborers. Cut the ball and chain of all three. When you talk to Sam in the southwest corner, he teaches you the melody that restores memory. He says to play the melody for Lance, who lost his memory. Then Sam asks to have the melody of wind and the prison keys. You automatically give them to him (no need to play the melody). Now you can leave the mine.
When you go back up to where there is a sign in the ground near the door that you unlocked, go west, then south to leave this area. From there, just follow the path to exit the mine. Outside, choose to go back to Freejia.
Go to the hotel and go up to Lance, then play the Memory Melody for him. Lance remembers everything, and all the others share their memories, and Will is moved to tears.
When you are ready, talk to Erik to learn that Will's cousin Neil is in the woods nearby. He is an eccentric inventor who flew in an airplane. You automatically go to Neil's Cottage.
Check on all of the inventions in the room. Then talk to Neil. He says that the world's ruins are in the shape of the constellation of Cygnus. He also mentions that the Tower of Babel is in the middle of the ground painting of the big white bird. Neil asks if you are ready to go to the ground painting. Say yes and you will automatically go to the Nazca Desert.
Neil will encourage you to walk around and see just how big the ground painting is. The others stop and look at the stones that are part of the ground painting. Go to the southwest corner of the area to find a Moon Tribe Shadow. Follow it until it disappears. Then go back up and talk to Neil. You all realize that the stones in the ground painting match the constellation of Cygnus. Then you try to figure out where in the painting the red star would be. The correct answer is Left Foot. Follow everyone to that spot, then check in the middle of the area that you are all looking at. Suddenly you are transported to the Sky Garden.