Terra is back on the team, and with her she brings the hope that the Returners can reach out to the espers, in their own world, and bring them into the war against the Empire. First, though, she and the other Returners must bypass a cave filled with ancient dangers. Sounds like fun.
Fly the Blackjack east of Albrook. Enter the Imperial Base, previously bustling, and you'll find the place abandoned. There's nothing to look at here, besides the single building in the middle of the base (and its most interesting contents are locked away at the moment), so take the northern stairs out of the base and back onto the world map. You'll find the Cave to the Sealed Gate.
There are two different segments of enemies in Cave to the Sealed Gate, though virtually all of them are undead and share a weakness to restorative spells. Many of the enemies down here are weak to ice and water attacks, making Shiva and Mog's Water Harmony excellent weapons for getting through the cave. Despite being undead, most enemies either resist or absorb fire attacks. The only exception to this is the Zombie Dragon.
You'll find a treasure chest immediately inside the entrance of the Cave containing an Assassin's Dagger. Go south and you'll find a Kazekiri in the next room. This next section features a small puzzle wherein sections of walkway will change places every few seconds. Keep an eye on the changing sections to cross safely. The chest south of the entrance contains Heiji's Jitte, a relic for Setzer that changes Slots into Gil Toss. The chest east of the entrance contains an X-Potion. Make your way to the exit in the south.
You're now in the cavern proper, a length series of rocky paths leading far to the east. Start by taking the upper path to the southeast. You'll find a Hi-Ether at the end. Backtrack and take the lower path. Soon you'll reach a ramp. Go up it and hit the switch on the next bridge. You'll drop through the bridge and land below. Go west and north, through the hole in the rock, to find a Genji Glove in a chest.
Backtrack and continue east. There will be a bridge ahead, as well as two switches on the northern wall. The right switch will bring a Ninja down for you to fight, and he'll drop a hint as he dies. The left will open up a chamber with a save point and a Tent. Heal up as appropriate and return to the bridge outside.
Hit the switch halfway across the bridge to create a set of stairs to the east. Go down the stairs and check the chest on the left side of the clearing for a Hi-Ether. Head to the left wall and you'll find a path through it, leading north. Emerge on the next section of the cave and check the chest on the small island here for an Elixir.
There are two switches to the east of here, though neither is helpful. Go south and west to find a switch on a raised hunk of rock. Touching it will open a chamber in the wall nearby. Inside are two Magicite Shards (they summon random espers when used in battle), a Hi-Ether and an Ultima Weapon. The Ultima Weapon's power scales with the bearer's HP, so give it to the person with the most health. The Ultima Weapon will be useful for the rest of the game.
Go back out and head east to the second bridge. To the north is a switch that will shift a nearby island. Run across and hit the next switch to shift it again. This will open a path to a treasure chest containing a third switch. Don't bother with this switch - it will just block your path forward. Cross the bridge to the north to find another Magicite Shard. Beyond this, to the south, is the room before the end of the line.
Approach the large, ominous cave in the rear of this final area. You'll find the Sealed Gate itself. Terra will attempt to make contact, but Kefka, having followed the group, will intervene. You'll have a short, one-hit battle against him before Terra can open the Gate, letting a mighty stream of espers out into the human world. They'll rush away, taking Kefka with them.
In the aftermath you'll be back in the previous corridor. A new exit will have opened back to the entrance of the Cave. Exit and return to the Imperial Base. One of your team members will show up and tell you where the espers went… Vector. Uh oh. You'll get back on the airship, but the espers will show up and knock the Blackjack out of the sky.