The house Impa told you to visit is covered by a rift, so Tri helps you safely enter the rift to look for their friends.

Explore the Stilled Suthorn Forest

If you want to exit the rift, you can walk on the glowing spot and press A to return.

Otherwise, go north. There will be a cutscene. Go east and jump over the gap or create a bed to bridge the gap. Follow the path north. There is a dark floating monster. You can create echoes of monsters to attack it. Spear Moblins work well. But beware: after you start attacking the dark monster, it will fly onto you and learn monster echoes that you know, then it will turn into one or more of those monsters and attack you.

If you defeat the dark monster, it doesn't drop an echo to learn, so just go east. Create beds to make a bridge over the floating trees, or just jump on the trees to reach the other side.

You will find a petrified person there. There will be a cutscene.

Go east and enter the cave.

First Cave in Stilled Suthorn Forest

Create a Spear Moblin and pick it up, then go to where there are two dark Keese. The Spear Moblin can throw spears at the Keese if you target the Keese with ZL.

Then go to the northeast corner of the room and step on the switch to open the door. Go to the door and go through it.

You need to defeat all the dark Zols to continue. Try creating a Caromadillo and targeting the Zols to make the Caromadillo roll through them.

After the Zols have all been defeated, the north door will open. Break any remaining crates in case they contain items, then go through the north door.

Stilled Suthorn Forest Part 2

Go northwest. You can create beds to help you get over the gaps. Go north to find another dark floating blob. As before, if it touches you, it can learn monster echoes from you and create dark versions of those monsters to attack you. Create monster echoes of your own to try to defeat it.

From here, you can go southwest, northwest, or northeast.

Southwest

If you go southwest, you find a ladder down into a cave. Enter the cave, and go partway down the ladder. Create a Caromadillo to defeat the Spear Moblin down below. Then go up the second ladder and open the chest (stand in front of it) to get 50 rupees.

Then create a trampoline next to the tree to jump on top of the tree, then create a couple of beds to reach the island above, and open the chest to get 20 rupees.

Then go back to the big island where you fought a floating dark monster.

Northeast

If you go northeast, you can create a bed to reach the second island, then you can create a bed and us it to get on top of the sideways trees. Jump up the sideways trees, then jump onto the top of their island. Then you can create a bed to reach the blue rupee on the island above.

Go back to the big island where you fought a floating dark monster.

Northwest

If you go northwest, there will be a brief cutscene.

Continue following the path, then enter the building that you find at the end. This is the Suthorn Ruins.