After the scenes, you’ll end up in some kind of watery temple. Upon regaining control, go forward for a bit. On the left side, you will find something on the ground, called the Al Bhed Compilation Sphere. If you don’t know what Al Bhed is, it’s basically a dialect in the game and throughout the story, you will be able to find books that help you translate a letter from it (it basically jumbles the letters around). This Compilation Sphere lets you load up a previous save, for example one where you found all of the books, to play through the game while being able to read all of the Al Bhed language you find.
Before moving on, there is a chest south of the sphere, which contains 2 Potions. Going back to Al Bhed, one such example of the language is on the right side of this initial area. There will be a glyph on the wall that you can’t really read, as it’s in Al Bhed. Of course, if you had loaded a save with all of the books, you would be able to read it. Behind the glyph will be another chest, with this one containing 200 Gil. Exit to the north to enter the next area, with a Save Sphere right next to you.
In this area, go along the path and left to a Hi-Potion , then along the main path northward to a scene. The path will fall out beneath you, dropping you into the water. Swim forward a bit to engage in battle with some Sahagins. Defeat two of them and when there’s one left, something a little bigger decides to make an appearance.
BOSS - Geosgaeno
Unfortunately, you won’t be able to win this battle against Geosgaeno, so don’t worry about actually healing. You can attack the thing, but the boss will just smack you with a physical attack on its turns, which will cut your HP in half, similar to Sinspawns Ammes’ Demi spell. So, just keep fighting or doing nothing and on the boss’ third turn, a cutscene will trigger.
After the cutscene with Geosgaeno, you will regain control in a hallway, so exit to the north to enter a large area. There will be a campfire in the middle of the room, but it’s not lit and you don’t have any items to spark another fire. Go to the northeastern door, which is locked, and check a slightly hidden area to the right of it to find a chest with an X-Potion inside. If you look at the minimap, there is another door to the north, bringing you to a staircase. Before doing anything, though, check the southern side, right by the entrance, for a chest with an Ether in it.
Climb the stairs and hug the right wall, where the camera will quickly pan to something on said wall. Examine it to find a Withered Bouquet, which will be some nice kindling to get a fire started. Pass through the doorway nearby and follow the path to the end, where another chest with a Hi-Potion will be located. Loot it, then return to the main area. On the southwestern side of this area is a Save Sphere, in case you need a heal, with another door right next to it. Enter through this door and check the debris in the middle of the room to find a Flint, which will become useful in trying to start a fire. Double back and examine the campfire to get a little toasty, but it seems to have attracted some unwanted attention.
MINIBOSS - Klikk
The Klikk isn’t technically a boss (the description at the top when hovering your cursor over it doesn’t say boss), but will be treated more like a mini-boss here. It doesn’t have much of an offense, mostly a basic physical attack for 100~125 damage or so. While it’s not a lot, you can actually lose to this creature, so you will want to keep an eye on your HP at all times. If your health dips below 200, then you will want to toss a Potion on yourself (they heal for 200). Anyhow, there’s not a lot you can do at this point since you, so you’re basically left to just using the Attack command.
Halfway through the fight, there will be a scene in which some other people will arrive. One of them will ally you for now. This character mostly specialises in Stealing from the enemy, then can use the Grenades she steals to damage them. Grenades ought to do around twice as much damage (or more) of Tidus’ normal attack. Unfortunate for you, Klikk’s HP seems to reset here, so your battle is just beginning. Whether you want to use the Grenades is up to you (you’ll be required to use one when the mysterious helper arrives), but be sure to keep your HP up regardless.
There will be a few scenes after the fiend has been taken down, with you then being brought to a ship.