Deist Cavern is another five-floor dungeon, this one split into three distinct paths. Conveniently, this split happens right on B1. The first time you enter, you'll be looking for the Pendant, which is quickly found on B2. Another of the three paths leads to Treasure, while the third leads to a spring that you'll need to return to shortly.
Deist Cavern B1
The Monsters on B1 are mostly similar to those found on Deist Island. There are no more Stalactite groups, and Hill Gigantes are more common. The rare new Monster are Yellow Souls. They absorb every Element but go down quickly to physical attacks with only 20 HP (but 25 defense). Split up your attacks and thin their ranks quickly, because they will repeatedly cast Fire 5 until you do.
B1 is a simple and largely open floor. The entrance is in the northwest corner, and there is a staircase in each other corner. The Pendant can be reached via the northeast stairway, so head due east from the start the first time you come here. You'll find a chest with a measly 30 gil along the way. Examine the body on B2 to receive the Pendant. From there, you should return to Castle Deist to receive your next objective.
The southeast stairs on B1 will lead to the spring that you'll be sent to shortly, but there is no advantage in going there early. The southwest stairs, on the other hand, lead to a linear path down to a Treasure cache on B5. You'll find a Potion along the way on B2, but at the end you will find four chests with rarer goods. The trio of chests contain a Flame Bow, Sage's Wisdom, and Saint's Spirit. The lone southern chest contains the Knight's Armor but is guarded by a group of Adamantoises. As ever, Ice is your best offense against them. Whether to delve this Treasure path now or later is up to you.
Back to Castle Deist
Pendant in hand, return to Castle Deist speak to the dying Wyvern. It will give you the Wyvern Egg and ask you to incubate it in a spring in the lower depths of Deist Cavern. With the Wyvern Egg in hand, return to the Cavern and prepare for a longer trip than your previous one.
Deist Cavern B2
The new enemy on B2 is the Revenant, a powerful but vanilla Undead creature that is most notable in that it will likely survive a single group casting of Fire. Their attacks are only slightly more intimidating than most of the other Monsters in the Deist Cavern. The exception, of course, is the Hill Gigas, groups of which are even more common on this floor than on B1. They remain the main threat to both your Characters' HP and your MP resources.
The southeast stairs from B1 lead to the same corner of B2. You'll have to head north at first, then west. The path will branch several times. The northern route leads to a suit of Mythril Armor, while the first southern route leads to a Mythril Helm. The second southern route leads to the central area of B3, but this is a false path—there is a bridge on that floor that will collapse if you try to cross it, leaving you on B4 on the treasure path. You can use Warp to return to the broken point on the bridge, allowing you to cross it. This area also holds two chests to the north. You'll first find a Mythril Gloves, then a Mythril Mirror. This might be the only time you ever see this item, which will make the user fully immune to all black magic Spells for one battle.
Continuing west past the first set of stairs to B3, path finally splits in two one last time. This southern route yields the Mage's Staff, which is guarded by non-threatening Green Souls. These odd creatures will repeatedly cast Cure 6 on the party until they run out of MP. When you're satisfied with their work, defeat them with physical attacks. The final, northern path leads to the true route to the spring on B3.
Deist Cavern B3
Gone are the groups of middling Undead on B3, replaced by more Yellow Souls and Revenants. The new rare encounter on this floor is with a group of Ogre Chiefs and Ogre Mages. Ogre Chiefs are yet another beefy 300 HP Monster without any Elemental weaknesses. The Ogre Mages may still be a better primary target due to the possibility of casting Blink or Status Ailment Spells, combined with their lower HP. Fortunately, both types of Ogres have multiple noteworthy rare drops.
The path to the spring on B5 starts in the southwest corner, and proceeds north. Do not try to cross the bridge to the east, or you will fall to the treasure path on the floor below, but you can find a Mythril Shield just to the east of your starting point. The northern path takes you all the way to the north end of the floor, across to the northeast corner, and then south to the stairs down to B4. The long walks on each floor will make this a treacherous expedition, so don't hesitate to Teleport out and heal up if you have to.
Deist Cavern B4
For a mercy, you'll find no more Hill Gigantes on B4. They've been replaced by a far less dangerous encounter with an Adamantoise or two, sometimes accompanied by Dual Heads and/or Stalactites. At this point, the encounter you least want to see is that against a group of Yellow Souls, though this remains the most common encounter on the floor.
The main path here is much shorter than it was on the previous floors, looping around north, west, and then south. You can pick up an Acid Phial along the way. This single-use Item casts Poison Cloud 16 on all Monsters when used. There aren't a lot of Monsters vulnerable to Poison, so you may as well use it on whatever seems appropriate. Despite the powerful effect, Acid Phials are only worth 300 gil.
Deist Cavern B5
B5 has arguably the least dangerous encounters in the Deist Cavern. Yellow Souls are gone, replaced by a rare encounter with Gigantoads. While these are powerful, Poisonous attackers with 450 HP, they are vulnerable to Ice and can be defeated quickly with the Blizzard Spell.
The layout of the floor is quite simple. Head east and you'll come across a set of four doors. The third from the left leads to the boss, while the other three each lead to trap rooms. The boss in question is a group of 1–4 Chimeras. Exactly how many appear will largely determine whether this fight is a cakewalk or a nightmare. They have no Elemental vulnerabilities or resistances, and have only 540 HP apiece. While their defense is high at 50, they are Magic Beasts and therefore vulnerable to the Demon Spear and Wing Sword, both Weapons you're likely to be using already. Chimeras spend about 1/3 of their turns using Blaze 7, and the damage will add up if you're facing several of them. Blink can help, as can Ice Shields.
Once the Chimeras are defeated, head over to the spring and use the Wyvern Egg. With that, your work here is done. You can Teleport to the surface and head back to Altair at your leisure. The mother and son back at Castle Deist will acknowledge your efforts if you stop in to speak to them first.