After Tri helps you enter the Stilled Suthorn Forest in a rift, you reach the Suthorn Ruins.
Learn a New Ability
Check on the cactus-like object to create a waypoint. Then lift and throw the pots for a chance to find items. Then go through the north door.
Walk up to the big rock that is blocking your path to the button until there is a cutscene. Then face the large boulder and press X, and there will be another cutscene. Tri will bind you together with the large boulder, and you can walk to the left to move the boulder onto the button in the floor. The door will open and you will automatically be unbound from the boulder. Go through the door.
Get the Dungeon Map
Face the large boulder and press X, then move to the left and press ZR to release the boulder. You will see a ladder in the ground where the boulder was. Before you go down the ladder, stand on the ramp in the north part of the room, face the boulder, and press X to bind to the boulder. Go up the ramp to move the boulder out of the pit that it's in. Press ZR to release the boulder, then stand southeast of the boulder and press X to bind to it again, then go up the ramp to put the boulder in the gap between the ramp and the northeast ledge. Press ZR to release the boulder, then walk across it, jump up onto the ledge, and go through the door.
Stand at the lowest part of the ramp and face the buried treasure chest, then press X to have Tri bind to the buried chest. While bound to the chest, walk up the ramp to pull it up out of the ground. Open it to get 5 portions of radiant butter.
Go back through the door, then go down the ladder that was beneath the boulder.
Go to the right, and climb up the string of spider silk that is hanging from the ceiling. Go to the right side of the solid ground, and create a monster to defeat the spider. It is only vulnerable when it is on its back, so it might take several tries to defeat it. After it is defeated, go down below and approach the shimmering floating monster, and press ZR to learn the Strandtula echo.
Whenever you create a Strandtula echo, it automatically creates a strand of spider silk above itself, attaching to the ceiling. Create a Strandtula next to the ledge above you, then jump on the Strandtula and climb up the silk to reach the ledge. Do the same thing beneath the gap in the ceiling to reach the treasure chest. Stand in front of the treasure chest and open it to get the Dungeon Map.
Remember to eat the food in your inventory if you lose health.
Learn More Echoes
Go down to the lower level and go to the right, and create a Strandtula. Jump on it and climb up the silk to reach the upper-right ledge. Go up the ladder.
Create a Strandtula next to the higher ledge on the left and climb up the silk to reach the higher ledge. Then go through the west door.
There is an Ignizol to the south, so if you haven't learned its echo yet, you can create a monster echo to defeat it so you can learn the Ignizol echo.
Go north and step on the platform in the middle of the north wall to go up.
Go north, and you will encounter an armored enemy. Create a Spear Moblin to defeat it. Walk up to the shimmering, floating monster and press ZR to learn the Darknut echo. If you lost health, remember to eat food from your inventory to restore hearts.
The path to the east is just a dead end, so go west and create a trampoline to jump on top of the stones. Go west, and go through the door.
Before you go down the ladder, walk up to the torch, which is shimmering. Press ZR to learn the Brazier echo.
Then go down the ladder.
Go down to where there is an unlit torch, and create a Brazier next to the torch, or create an Ignizol and throw it at the torch to light it. Then go up the ladder.
Go to the right, where a boulder is blocking your path. Press X while facing that boulder, then walk to the left to make the boulder float above the gap. Then press ZR to make the boulder fall through the gap. Jump over the gap, then climb up the ladder.
Create a Caromadillo to quickly break the wooden boxes, or create a Brazier or Ignizol to burn them. Then go to the left and create a Brazier near the unlit torch. Face your Brazier and press X to bind to it, then jump straight up to light the unlit torch. This opens the door. Go down the ladder, then go to the right and go up the ladder to the next area.
Check on the cactus-like object to create a waypoint.
Go to the right, where one of the statues has a shield. Face the shield and press X to bind to it. Walk south until the shield comes off. The door opens. Go through.
Create a Spear Moblin and press ZL to target the dark Keese. When the Keese are gone, go to the southwest part of this room. Create a Strandula near the tall wall, then jump onto the Strandula and climb up the silk to reach the top of the wall. Go to the left and create another Strandula to reach the treasure chest. Open it to get 50 rupees. Then create another Strandula to get back on top of the wall on the right, then go to the right until can go north.
To the north, there are two Darknuts. You can create another Darknut to defeat them, or create a Spear Moblin.
Then go north and up the stairs, and you will have to fight an enemy.
Dark Swordsman
Create two or three monsters to fight the dark swordsman. If the swordsman has his back turned, you can create a monster behind him to hit him while he's distracted by a monster in front of him. Ignizols are especially useful because the swordsman can't defend against being burned. Keep running around the room to keep your distance from the swordsman.
After a while, the swordsman will change, wearing a blue tunic instead of a green. His defense is higher. After you defeat this form of the swordsman, there will be a cutscene.
Afterward, go up to the sword and press A to check on it. You will receive the Mysterious Sword. There will be a cutscene. When prompted, press up on the directional pad to raise the sword. There will be a cutscene.
From now on, you can press up on the directional pad to transform into swordfighter form. While in swordfighter form, you can press Y to swing the sword, hold ZL to raise the shield, and jump higher than usual when you press B. If you hold the Y button, you will charge up a spin attack, and when you let go of Y, you'll perform the spin attack.
You can only stay in swordfighter form for a limited time. There is a countdown timer in the corner, but you can collect floating blue energy to add more time to your timer. You can also switch back to normal form by pressing up on the directional pad.
Go north and hit the dark membrane with your sword until it disappears, then go down the ladder.
Get New Equipment
Go down the ladder to the lowest level, then go to the right to find a large boulder. Stand next to it and press X to bind to it, then go to the left to pull it toward the lower part of the floor. When the boulder is above the lowest part of the floor, press ZR to drop it into the pit. Then go to the left and go up the ladder to the nearest level, and jump down onto the big boulder. Create a Strandtula on the right side of the boulder and climb up the spider silk.
Go to the right and create another Strandtula to reach the treasure chest on the right. Stand in front of it and open it to get a Heart Pin. You can go to the + pause menu to equip this to make hearts appear more often.
Get a Key
Equip the Heart Pin now (unless you want more of a challenge), then climb the short ladder and go to the left to fall onto the boulder that you moved earlier. Go to the right and go down the ladder.
Go north through the door, and collect the floating energy to add to your swordfighter gauge. Go north and there is more blue energy, as well as a waypoint that you can check on to add it to your map.
Go to the southwest corner of the room and go through the door.
There is an enemy just beyond the door. Press Up on the directional pad to switch to swordfighter form, then quickly hit the enemy with your sword. Go to where it fell, and you can learn the Deku Baba echo.
Gather more blue energy from this room (the energy can go through walls, so just try to get near it to make it come to you, or make a trampoline if you need to jump higher to reach it), then switch to swordfighter form and cut through the dark membrane on the left side of the room, then switch back to normal form (press Up on the directional pad) and create a monster, such as a Darknut, to defeat the Deku Baba.
Then go to the northeast corner of the room and switch to swordfighter mode, jump up the steps (you can reach them because you jump higher in swordfighter form), and swing your sword to defeat the Deku Baba. The doors will open.
First, go through the west door. Face the green statue behind the bars, then press X to bind to the statue. Go up the ramp to push the statue onto the high ledge, then walk sideways without going south, to move the statue toward the right, along the ledge. Then when the statue is next to the east wall, go south until the statue is on the button. Press ZR to release the statue on the button. Then open the chest to get 50 rupees. Go back through the east door.
Go through the north door. There is a Deku Baba behind a metal fence. You can just face the Deku Baba and press X to bind to it, then walk south to pull it until it is defeated. Open the chest to get a Small Key.
Learn a New Skill
Go through the south door, then through the east door, then go up to the locked door in the east wall, and press A to unlock it. Go through.
Go to the right side of the room and face the statue that is up against the east wall. Press X to bind to it, then walk west and then north to put the statue on the green tile on the floor, so that the four statues are in a square formation. The gate will open. Go through, and go down the ladder.
Go to the right and wait for the moving platform to come close enough that you can jump onto it. Then ride the moving platform to the right side and jump off there. Go through the opening in the east wall.
There will be a cutscene, and you will learn about Reverse Bind. If you bind to an object, you can then hold R to move along with the object that you're bound to.
Get the Big Key
Tilt the left control stick up so you're facing the moving platform up above. Then press X to bind to the platform. The platform won't move while you're bound to it, but if you hold the R button, you will follow the movement of the platform instead. Hold R while bound to the platform, and let it take you to the other side, then press X to release the platform. Then you can climb up the ladder and ride the platform to the left side.
Go up the ladder, then get on the moving platform here. Then, when prompted, hold ZL to target the platform above you, and press X to bind to that platform, then hold R to move along with that platform. Let it take you to the right side. Then go up the ladder and ride on the moving platform to the left side.
Use the platform above you to reach the right side, where you can stand in front of the chest and open it to get 20 rupees. Then climb up the ladder while still bound to the platform, then release it and jump onto it to go back to the left side. Go to the ladder on the left and climb up.
Bind to the statue behind the bars, and push it up the ramp around the big chest. Then go to the left to bring the statue near the button, and jump up while moving, to make the statue go onto the button. The bars will go down. Go north and open the big chest to get the Big Key.
Press - to open the map, and warp to the waypoint to the left of where you are. Then you can go up to the Big Door and press A to unlock it. Go north through the Big Door. There will be a cutscene, and you will have to fight the Seismic Talus.