The cavern opens up after defeating Defender X in the Calm Lands. Before diving into the cave, it’s a good idea to make note of how much money you have, since you will be using it at the end. If you don’t have at least 210,000 gil or so, then you will probably want to put things off until later in the game. Should you have the money, then you’re free to go in. The first big room of the cave has nothing, so continue into the next corridor, where you should see a path branching off. At the end of this short path is a chest that contains a Megalixir.
Continue into the next room for a short scene, then head to the next intersection. The very short path to the north has nothing, but if you veer to the right, you will find another chest with a Lv. 2 Key Sphere. Take the other passage to the west, where you will come to more splits. East has nothing, but the northern path leads to a dead end with a chest, containing a Fortune Sphere. At the last split, there will be a chest straight ahead that has 2 Mega-Potions. The little dead end to the west has Al Bhed Primer Vol. XXV laying on the ground. The final corridor will have a Save Sphere and when you enter the room at the end, you will be forced into a boss fight.
BOSS - Yojimbo
Yojimbo is a pretty simple boss, as his attacks depend solely on how full his Overdrive gauge is. First and foremost, the gauge only fills very little upon being targeted, as well as when he attacks himself. At the beginning of the battle, Yojimbo will only use Daigoro to attack, which will have him send his dog to physically attack. Once the gauge is about 25% full, he will add Kozuka to the list of attacks, which does a little more damage than Daigoro. The last attack in his arsenal, Wakizashi, comes out at around 50% Overdrive gauge.
There is a good chance that you might not see Wakizashi, though, as Yojimbo only has 33,000 HP here. He doesn’t give AP nor drop items, so there is no point in bringing in everyone to take an action. Should the unthinkable happen and his Overdrive gauge gets filled completely, Yojimbo will use Zanmato, which will hit your party for 9,999 damage, meaning they will be instantly defeated. You can use pretty much whatever you want in terms of offense, although his physical defense is a little better than his magical defense.
Upon defeating Yojimbo, the teleport pad in the room will be activated. Before heading to the Chamber of the Fayth, step on the pad and press left, then X to get sent into an isolated room that has a chest (Flexible Arm). This is a nice weapon for Rikku, as it has four open slots for customization, so make sure you save it for later. Return to the main room and teleport to the right room this time, for two more treasures, which are a MP Sphere and 2 X-Potions. You can now enter the Chamber of the Fayth, where you will find Yojimbo, but things are a little different than previous Aeons.
In order to recruit Yojimbo, you will have to offer him some money. Before doing that, he will ask you a question and if you want the cheapest option for an offer, pick the third one. Yojimbo will start the offer at 250,000 Gil, but you can get him down some, so offer 125,001. The asking price should go down and you should offer something in between the above amount and the asking price. It’s a bit weird, as usually half of the asking price, plus one gil, should be enough to get him down to around 200,000 Gil. He can go down to around 192,000 or so and if he refuses you, just go back in and try again. Eventually, you will get Yojimbo as one of your Aeons.
That’s about all for the Cavern of the Stolen Fayth, so take the teleporter in the final room to return to the entrance. Note that if you exit the cavern, then re-enter, there’s one more surprise waiting for you. Head to the final room and attempt to leave via the normal path, only to have a summoner conjure up Dark Yojimbo, one of the optional bosses designed for the very endgame. You aren’t forced into the fight, though, so you can use the teleporter to return to the entrance and leave.