When you gain control of Tidus, Wakka and Rikku, you will be right by a Save Sphere, so use it to save your game. Across from it is a chest, but this isn’t a normal chest, as it functions as a shop, in case you need any items. When you’re ready to continue, swim into the next large area and keep going to the end, where you find another Save Sphere. As you swim into the open section, a familiar face pops up, triggering a boss battle.
BOSS - Evrae Altana
Evrae is back, but it is now an undead creature this time, which can make it an extremely easy boss. If you wish to fight it normally, let’s go over some of the things it does. It still has Stone Gaze, which behaves a little differently underwater, as it will instantly shatter the character. This means they will be ejected from battle, so they will receive zero AP at the end. If this is Rikku, then that can be troublesome, as she is probably the only one who has access to Use and can heal the party with Al Bhed Potions. Of course, if you have Stone Ward on your armor, then there’s a chance you won’t get petrified.
The only other attack that Evrae Altana has is Photon Spray, which hits random characters eight different times. It will also counterattack any attack you do with a physical attack, so you will need to keep an eye on your health at all times. So, fighting it normally is one way of doing things, but there are other ways. You should see that there are Trigger Commands in this battle, which is to open locks. Doing this will move the battle to another area, as well as revive/bring back other party members that may have been petrified and shattered.
Of course, doing this will not only get you away from Evrae Altana, but it will cut you off from a Save Sphere and some treasures. Also, there is no reason to do this, either, as there is one more way to defeat this boss. As a Zombie/Undead, you can simply use some restorative items to do away with the boss in a super easy way. In fact, two Phoenix Downs or X-Potions will down Evrae Altana.
Upon defeating Evrae Altana, if you didn’t open any locks up during the battle, you can go back to the Save Sphere right before it. Also, there will be no more random encounters down here, so you don’t have to worry about more fighting. Press forward and open the chest for a Rematch weapon for Wakka, right before the turn. This weapon has Evade & Counter on it, which is pretty nice, so it’s a good idea to keep it, even though it doesn’t have any other open slots. Right before the next open room, you will find one more chest, which contains an Avenger. There’s nothing left down here, so swim to the end to meet up with the rest of the party.