The castle Pandaemonium has arisen in place of Castle Palamecia, along with the resurrected Emperor. The people in Castle Fynn mention that you cannot enter Pandaemonium directly. Hilda will teach you about the [Jade Passage], and direct you to Mysidia to learn more. Leila reveals the actual location of the Jade Passage, near the strait east of Mysidia. (Gordon mentions that is is near Kashuan Keep, though "near" is somewhat of a stretch.) If you visit the library in Mysidia as Hilda suggests, you'll learn some more context about the Jade Passage and Pandaemonium, but at this point all you need to do is enter it and defeat the Emperor. This is easier said than done, of course.
Jade Passage B1
While the Jade Passage only has six floors to delve, you'll reach Pandaemonium at the end of it, adding 8 more floors to your trek. This is by far the longest dungeon sequence in the game, but you have a few things working in your favor. A lot of your exploration time will be spent finding Treasure, so return trips should be considerably faster. If you Teleport out, you can immediately ride the airship to any town in the game to rest up. (I recommend Mysidia, which is close and allows you to land adjacent to it.) Don't hesitate to end a run and start a new one. You'll be building up stats as well as equipment as you do so.
The Monsters on B1 are similar to those outside, but it's unlikely you've faced those unless you went out of your way. Hecteyes, Salamanders, and Great Malboros all appear regularly in groups. The fourth common encounter is with a White Dragon just like the one that was guarding the Black Robe in the Mysidian Tower. While not technically a common encounter, you'll see Abyss Worms as frequently as the other Monsters listed here, sometimes accompanied by Great Malboros.
What all these encounters have in common is Monsters with a lot of HP (over 1,000 in each case). Hecteyes are spellcasters that resist every Element and can devastate your MP with Osmose 10 or turn your Characters against one another with Confuse 16. Salamanders are similarly tough foes, but with an exploitable weakness to Ice. Their Blaze 16 ability will make you thankful for Fire resistance. Abyss Worms are somewhat similar in that they have a nasty Cyclone 13 attack, though you cannot resist that. They also have no Elemental weakness, so stick to non-Elemental Spells and physical attacks. Great Malboros are the most like the later games' Malboros. While they don't use Bad Breath (we'll get to that), their attacks inflict every temporary Status Ailments at once and can wreak havoc on front-line fighters with insufficient Evasion. This almost certainly includes Leon. They have no resistances, so use your best attacks to take them out before they cause problems. White Dragons take a while to kill due to their 120 Defense and 2,500 HP, and Icestorm 16 is no joke. Hit them with Fire or even the Blood Sword.
The most dangerous Monsters on B1 are fairly rare, but you'll see more of them as you descend. These are the Chimera Sphinxes, upgraded versions of the Rhyos. They also use Blaze and Bad Breath, but at levels 16 and 10 rather than 7 and 6. Anyone without resistance to Matter (granted by the Black Garb and White Robe) is very likely to be turned to Stone by Bad Breath 10. Their other attacks aren't too threatening (and Dispel doesn't do anything at all), and fortunately they lack resistances or high Defense. Even their Magic Resistance is low, so Status Ailment or KO Spells are a strong option. The rarest encounter on B1 combines Mega Parasites and Ice Lizards. The former can cause problems with Osmose 10, but overall this is one of the less dangerous encounters on the floor.
The first floor layout is pretty simple, and you'll notice this dungeon sports a very unique look for a final dungeon. It's bright and downright tropical. There are three doors on the floor, all in the eastern half. The northern and middle doors lead to trap rooms, while the door in the southeast corner leads to B2. You can find the Diamond Mace, the most powerful staff in the game, by following the northern branch of the water. There is a Hellfire stashed in the southwestern corner of the floor, which you can collect without going too far out of your way toward the stairs down.
Jade Passage B2
B2 contains a lot more Abyss Worms than B1, but no more Hecteyes. Once again, the rarest encounter is one of the lesser ones. In this case it is a combination of Great Malboros, Ice Lizards, and Pit Fiends. This eclectic group has a large variety of attacks at its disposal, but not much that's a threat to do serious damage.
Once again, the stairs are hidden behind a door, and all the other doors lead to trap rooms. In this case, there are five doors scattered along the floor. The layout is a sinuous path, and the correct door is predictably the one at the very end of it. If you head north from the stairs, you'll see a second Hellfire a bit off the main route in the northwest corner. Follow the water east and then south to continue on. You'll see a chest on an eastern branch, this one containing an Antarctic Wind. When you reach the large waterlogged area, you can head southwest to find a guarded chest containing the best knife in the game, the Cat Claws. Note that in addition to high Power, they are effective against the very common Spellcaster Monster Family. The guardian here is the third dragon you've seen, the Blue Dragon. With 150 Defense and 3,500 HP, it's not a Monster to take lightly. You can use the usual high HP boss tactics, or exploit its weakness to Poison with Scourge. Its Lightning-Elemental Thunderbolt 16 ability can deal significant damage to anyone that doesn't resist it.
East of the large water area, you'll reach the edge of the floor. Head north and enter the door in the northeast corner to proceed to B3.
Jade Passage B3
You get a break from White Dragons on B3, but now you'll be fighting Abyss Worms almost 40% of the time. The rare encounter is an combination of Killer Mantises and Vampire Ladies. The Vampire Ladies can cause problems with Charm 9 (which fortunately targets a single Character) and Sleep 16, but the Killer Mantises should be a pushover by now.
B3 represents the first real departure in floor layout for the Jade Passage. You'll follow a narrow path across several tiers, winding back and forth across a waterfall. The waterfall tiles are damage tiles, which give a welcome reprieve from Random Encounters. You'll pass three doors on your descent, all of which are trap rooms. You'll also note a guarded chest on a plateau. This contains the Rune Axe, the best axe (and best non-sword Weapon in the game. Like the Cat Claws, it is effective against the Spellcaster family. It's not a bad choice for Leon if he doesn't need the Evasion bonus from the Defender. To earn it, you'll need to fight a King Behemoth. This fight is manageable if you get Blink up early, because its 8 attacks are powerful but it has no other means of offense. It does have 120 Defense to go with its 5,000 HP, but if you don't want to deal with that you can roll the dice on its highly volatile Magic Resistance and try a KO Spell like Toad.
The bottom tier of B3 is where things get extra tricky. While you can walk around the bottom of the waterfall, you can actually walk under it from the east to find a secret Spell Shop NPC. This shop sells Berserk and Haste, which until this point were only obtainable as drops, and you can also buy extra copies of Flare if you need them. Past the waterfall, you'll find a series of three doors. Two are trap rooms, and the door furthest east leads to B4.
Jade Passage B4
The bad news among common Random Encounters continues to grow on B4. Salamanders will no longer appear, but Chimera Sphinxes are now a common encounter instead. The rare encounter is with a powerful Green Dragon. The good news is that there's a lot of good Treasure to find on B4, including a whole Treasure room.
B4 has a wide open layout reminiscent of B1. You start in the southwest corner, and the door to B5 is across the way in the southeast. You don't want to miss the chests on your first trip, though. You'll find a Betrayal Fang north of the starting point, and a door leading to three more chests past that. This is the only room in the Jade Passage that doesn't contain stairs or is a trap room. Inside, you'll find the Aegis Shield, Dragon Armor, and Holy Lance. The Aegis Shield has the highest Evasion bonus of any shield, and resists the four Status Ailment Elements. The Dragon Armor complements it nicely, offering resistance to the four basic Elements. It is heavy armor, so watch your Evasion, but giving one Character both is as effective as giving them a Ribbon. The Holy Lance is the best spear in the game, though it's well behind the top Weapons in general. It is effective against Dragons, but don't use it as an item as it will break after casting Holy 8.
From the Treasure room, continue east to find Yoichi's Bow. It is guarded by the most powerful Dragon, a Red Dragon. This fight can be dealt with similarly to that with other Dragons, though this one has a weakness to Ice. Yoichi's Bow is the best bow in the game, and a massive upgrade in both Power and Accuracy over the Ice Bow. It's a good backup weapon for a back-row mage, but it sports the same 70% Magic Penalty as all bows so don't leave it equipped while casting. The stairs to B5 lie to the south of this final chest.
Jade Passage B5
You must be getting deep, as the tileset has changed from a flooded tropical cave to a narrow path over a yawning abyss. The Monsters here are arguably easier than those above. Large groups of Great Malboros will no longer appear, and the new random encounter combines Chimera Sphinxes with the far less dangerous Scissorjaws. Of course, you'll also be dealing with more Green Dragons on this floor, so the news is not all good.
The winding path is simple to navigate, and you'll find three treasures as you traverse it. The one door in the middle of the floor predictably leads to a trap room, while the one at the end leads to B6. In order, the chests contain an Hi-Potion, a Protect Ring, and a second Hi-Potion. Protect Rings are fantastic rear-row hand Armor, offering a lot of Defense and Magic Resistance without a Magic Penalty. They are made somewhat redundant by Ribbons, so prioritize mages who don't have one when assigning gear.
Jade Passage B6
As you near the end of the abyss part of the Jade Passage, it may seem odd that Abyss Worms finally decrease in their appearance rate. Hecteyes appear alongside them in the new rare encounter, but there's not much new to worry about here, except for the increasingly common Green Dragon. Once again, this is a very linear floor, though unlike B5, you'll find no treasure here. What you will find is a teleporter to Pandaemonium 1F. Just walk into the center of the final plateau to activate it.