After encountering Ultros on Lete River/Lethe River while trying to get Banon to safety, the group is separated. This page explains how to complete Terra's scenario.
Go to Narshe
You will automatically continue on the raft down the river, and there will still be random battles. During a battle, re-equip Terra with the Dirk/Dagger in her left hand if you equipped her with the Genji Glove relic earlier.
After the raft goes into a cave, you'll be on the world map. Go west, then north and into the mountain path. You'll be back in Narshe. First, go into the house near where you entered the town, and use the recovery water in the bucket. You can leave town if you want to save right away. Then, back in Narshe, go west from where you entered Narshe. Check the rock wall to reopen Locke's hidden passage.
Go into the mine and follow the straightforward path. When you are outside again, use the path on the left and go through the door to enter another part of the mines.
26. 1st Class/Valeor
27. Wild Rat
28. Dark Side
Follow the straightforward path through the mines until you see a spark of light travel through a maze. You need to follow the same path as indicated by the spark of light. If you go the wrong way, sparks of light will surround you, and if you don't hit the "tag" light, you fight Dark Sides, Spectres/Specters, and/or Rinns/Eukaryotes, and then you will be sent back to the beginning of the maze. However, this is your only opportunity to fight those types of enemies, and fighting certain enemies can be helpful later. You might want to go the wrong way on purpose for this reason.
After you get through the checkpoint maze, go through the door at the end. Go north until you see a door in the wall. Go through it, then follow the path to reach the Moogles' place.
Go to the north wall of the Moogles' place, then go west through the wall, and when you can't go any farther west, go south. There is a chest, but if you don't open it, you can get a Ribbon later. Go through the door south of the chest.
Go east, then south across the bridge (ignore the chests so you can get better items from them later), then go east, then south across another bridge. Save at the save point if you feel like it, then go south to exit the mines.
Go east along the bridge, and keep going east until you can go no farther. Then go south to enter Arvis's house. Go south and you will talk to him. After a cutscene, if you have completed all three scenarios, you will proceed to the cliffs of Narshe. If you have not completed all three scenarios, then you will have to proceed to either Locke's scenario or Sabin's scenario.