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Sealed Cavern

- Once your airship is ready so sail across lava, hop in and fly to the southwest corner of the underworld. Down here you’ll find a cave in the midst of a mountain range. Land and head inside. Assuming you fetched Luca’s Necklace from King Giott back at the Dwarf Castle you can enter the Sealed Cavern beyond.

- The enemies down here are not too bad overall, and fire will prove to be a better friend than most elements. Your primary nemeses will be Trap Doors, monsters that occupy the majority of doors down here. Fight one and it will spend four turns targeting, and then instantly killing, two of your characters. You can either take it down with your strongest attacks in that time, not easy, or, as the Final Fantasy Wiki suggests, cast Reflect on a character that has been targeted by the Door. Its Ninth Dimension attack will bounce back and kill the Door. If you don’t kill a Trap door in four turns it will turn into a random enemy, most often a Chimera Brain. These are painful beasts, and their Blaze attack can devastate your party. Kill ‘em quick with neutral attacks.

- Head south from the entrance. On the next island is a door, the first Trap Door, behind which you’ll find a Kotetsu and an Ether. South of this door you’ll find a Bestiary

- Continue northwest from here and you’ll find a Trap Door, behind which is a room containing a Hi-Potion, an Ether, a set of stairs, and two more doors. Predictably, these two doors are Trap Doors. Both chambers are empty. Pfft!

- B2. Head south down the left rope to find a Bestiary, then cross the bridge to the east. You’ll find a chest containing a Phoenix Down. There’s a line of Trap Doors to the north, six strong. The first on the right leads to an empty chamber; the second contains a Light Sword; the third contains a Fuma Shuriken, an Elixir, and a Light Curtain; the fourth is empty; the fifth contains a save point; and the sixth contains a Kotetsu and a Black Cowl. South of this sixth door is another Trap Door, and behind it an Ether and some stairs.

- There are chests in the next room containing a Hi-Potion and a Phoenix Down. The room on the left is empty, so don’t bother entering unless you want Trap Door experience. Go through the door in the south.

- B3. To the east, past a long rope, is a chest containing a Bell of Silence. The room beside it is empty. Climb down the rope and look on the right to find a Trap Door room, this one containing a Dry Ether and an X-Potion. Then return to the main room and head west to find more stairs. They lead to a save point.

- Continue south. Through two more rooms, one of which forces you to fight a Trap Door, you’ll find a Dark Crystal. Fancy. Head back out, though, and…

Demon Wall

This battle is largely a test of your damage output. Demon Wall starts off far from your group, and every few turns it will inch a bit closer. During this time it will smack you with weak physical attacks or occasionally paralyze one of your members. Not too difficult… unless you let it get close enough. Then the Demon Wall will start to use its instant kill move, Crush, which can quickly obliterate your party. Kill it before then by employing your strongest, fastest attacks to delay its turns. Normal attacks from Cecil, Kain, and Edge are recommended, potentially while Berserked, and Rydia can use her magic. If you picked up Leviathan before you can do a crapload of damage to Demon Wall each turn, preventing it from getting anywhere near you. Not a fantastically difficult battle, but painful for lower-level parties who are lacking in hard-hitting attacks.

- You can’t Teleport, so make your way back to the entrance of the Sealed Cavern. Upon arrival… well, Kain leaves the party again. What an asshole.

- Return to Dwarf Castle. Speak to the king and Cid will pop in to alter your airship. From this point on you can use it to move between the overworld and the underworld via the hole near Agart. The transit point in the underworld is to the northeast of Dwarf Castle.

- Your next destination is, officially, Mysidia. First, though, we have some side quests to complete, one of which will disappear if you don’t do it now.