After visiting Tzen and Mobliz, you should go to Nikeah next.
Go to Nikeah
Nikeah is at the northwest end of the thin, curved part of the continent. If you are going there from Mobliz, there is a Chocobo stable to the south of Mobliz, in the west part of the forest. Find it, and you can ride to Nikeah.
Buy what weapons and armor you can afford, then go into the Cafe. Talk to the bandits until they leave. Follow them onto the ship, then talk to a bandit and to the ship captain. After that, go back to the market, find Gerad (standing to the left of the armor shop), and talk to him. Get out of his way, then follow him. Keep talking to him and following him until you're on the ship.
In South Figaro, check all the barrels and boxes near the docks to find some items. Go to the Inn and go upstairs. Go to the rightmost room and talk to Gerad. He'll leave.
If you are running low on any items, buy more now. Then leave town and go west to the cave.
134. Nohrabbit/Nohrabbt/Desert Hare
135. Sand Horse/SandHrse
140. Dante (use Fenix Down/Phoenix Down or Bio)
Before you continue, consider equipping protection against Muddle/Confuse.
In the cave, talk to Ziegfried/Siegfried, and follow him. When the path splits, go east, then north, then west. At the intersection of the four paths, go north. Follow the bandits across the turtle (just press A when the turtle is aligned with the door).
Make your way west until you reach Basement 1. Go south and you will see Gerad asking if the guard is ok. Go up the stairs where Gerad went, then go down the stairs on the left. In the room with the old man, go down the stairs, then go north and down another set of stairs. Go south from there, and go through the door in the south wall.
In the room with suits of armor, go south to find four chests. Then go through the northwest door, then go up the stairs, and go through the nearby door to find a chest. Then go back the way that you came until you are in the room with the four suits of armor. Go through the top-middle door. Go north to the next room.
Heal up before talking to Gerad, and equip the RunningShoes/Hermes Sandals to protect against Slow. When you talk to Gerad, he will finally admit that he is Edgar, and he will join you. A boss battle will start.
The Tentacles will attempt to cast Slow on your party, and will then pull the Slowed character out of battle and drain his/her HP and MP for a while. Use Haste on your party members to make them immune to Slow. Use the Siren Esper to Silence the Tentacles.
Use Edgar's BioBlaster to inflict poison on the tentacles. After that, have Edgar use Drill. The lower-left tentacle can be killed instantly with the Death spell or the X-Zone/Banish spell. The upper-left and lower-right tentacles can be killed with Break, but Break has a low success rate. Use Bio on the upper-right tentacle. Use Ice 2/Blizzara on all but the lower-left tentacle.
When you are done, go through the door to the north, and check the armor in the middle to find a weapon. Then go back the way you came until you reach the room with the old man. Go to the right and the old man will take the castle to the surface. Go upstairs, then go north, then use the door on the left and go north to rest in the castle Inn. Go south again, and if you need to buy more items, and use the upper-middle door, then go into the rooms on the left and right to find merchants. Then go back and ask the old man to take the castle to Kohlingen.