Narshe Area Encounters
The Monsters around Narshe are similar to those found in the Jidoor Area, though they appear in different groups. You will find nothing but Crawlers in the wastelands, in groups of one, three, or four. The plains are filled with all of the creatures found in the plains and forests of the Jidoor Area: Mantodeas, Sprinters, Spek Tors, Geckorexes, and Reach Frogs.
Duncan's Cabin
In South Figaro, Duncan's wife indicated that Duncan was alive and at his Cabin north of Narshe. It's not easy to find, even with her directions. Look for five trees in a cross shape northeast of Narshe, and make sure to bring Sabin. Duncan will teach him the Bum Rush Blitz, an awesomely powerful attack that Sabin would not normally learn until level 70. It deals magic damage and ignores Mag.Def. Remember that its damage is influenced by Sabin's Mag.Pwr rather than his Vigor. Even if you've been focused on increasing his Vigor, the Bum Rush will remain his best single-target Blitz until you reach much higher levels.
Narshe
Several townsfolk have mentioned Narshe, which has been overrun by Monsters. It's not entirely clear whether the game "intends" for you to go there now, but the rewards are great and the levels of the Monsters are appropriate for this part of the game. The immediate goal is to find and recruit Mog and get the Tritoch Esper, but you can continue on to Umaro's Cave to obtain the first "secret" character, Umaro. You'll also obtain the Terrato Esper when you defeat Umaro. Note that you need to bring Mog to speak with Umaro before he'll join up. For this reason, you may want to take a party of three into Narshe so Mog can become a part of the active party without having to return to the Falcon first.
Most of the buildings in Narshe are locked. As the pickpocket implies when you enter the town, Locke can pick the locks to gain entry. The Classroom, Arvis's House, the Elder's House, and the Armor Shop remain open.
Narshe Town Encounters
There are three sets of Encounters to deal with in Narshe. In the town itself, you'll find several shades of earlier battles in the area. Red Wolves have extremely basic attacks and lack any specific vulnerabilities. They are surprisingly hardy, but their damage potential is low. They can appear in packs or alongside Nastidons, which are similarly basic creatures. Their attacks are slightly more powerful and they resist physical attacks, but are vulnerable to Fire. In addition to appearing alongside Red Wolves, Nastidons may rarely appear in pairs.
The other Encounter you'll have in Town is with a Test Rider. These are human foes that are vulnerable to Poison and the Man Eater. Their attacks are more powerful than the other Monsters in the area, especially their Gold Lance special. They will use Gold Lance in response to attempts to Steal, so beware if you have already recruited Locke. They'll sometimes counter other attacks with a basic attack of their own.
Once you reach the Mines, you'll start to encounter two new flavors of Mag Roaders. The large yellow Mag Roaders can cast Ice and Ice 2, while the small brown Mag Roaders favor Fire and Fire 2. They can also use Fire Ball when they die. Neither has any exploitable weaknesses, so just take them out with whatever powerful attacks you have.
The deeper mines and the Snow Maze feature Red Wolves as well as Psychots and Wizards in various combinations. Psychots can try to drain your MP with Mindshock, though they never use any MP so this doesn't work. They are vulnerable to Ice but absorb Fire. Wizards, on the other hand, will use a wide variety of Spells. Each round they can use one of two: Mute or Osmose, then Rasp or Stop, and finally Muddle or Sleep. They can also counter Magic by turning the attacker into a Zombie. They are highly vulnerable to physical attacks (the Man Eater in particular), but if you want to use Magic, stick to Thunder or Poison.
The Snow Maze also features an actual wandering Monster, but I'll get to that shortly.
Finding Mog
Mog is in the Moogle Lair where you might expect. The fastest way to get there is to use the secret entrance west of the town entrance like you did way back in the World of Balance. Follow this path a long way until you find a door in the southern rock. This leads to the West Mine Maze where you fought to save Terra oh so long ago, and the north exit from there leads to the Moogle Lair. Alternately, head past the door to the next door to its south.
Mog will join when you talk to him, and if he never joined in the World of Balance, you'll get a chance to rename him. Check the wall behind where he was standing for the Moogle Charm, a useful Relic that completely prevents random Encounters, but can only be equipped by Mog. If there's room in your party, Mog will join immediately, otherwise he'll head back to the Falcon.
There is a chest in the western part of the Moogle Lair, near the southern entrance. This held a Rune Edge in the World of Balance, but if you didn't open it, it will now contain a valuable Ribbon. This is not the only Treasure that's been upgraded. The two Treasures southeast of the Moogle Lair, a Sleeping Bag and a Fenix Down, have been upgraded to an Elixir and a Pod Bracelet, respectively. The Pod Bracelet is a rare but effective defensive Relic that grants the wearer permanent Safe and Shell effects. Unfortunately, the save point that used to be found nearby no longer exists in the World of Ruin.
Your next destination is the Cliff Top where the Tritoch Esper awaits. You can follow the southeastern path back to Arvis's House and from there back into town proper, or use Warp to leave and save. Either way, you'll want to follow the northern path to the Mine Road. Make a right at the North Mine Entrance, then follow the path all the way to the Snow Maze.
If you fight a battle with Mog in the Snow Maze, he'll learn the Snowman Jazz Dance, possibly the last one in his repertoire. Watch out for a fast-moving blue dragon in the area. This is the Ice Dragon, and it is not a minor threat. It's one of the eight legendary dragons, and you don't want to fight before you've prepared and saved. The save point at the north end of this area is active, and you'll probably be itching to use it. You can challenge the Ice Dragon or Tritoch first, but you'll probably want to defeat both. The Ice Dragon drops the Force Shld, one of the best shields in the game. Defeating all 8 dragons gets you the Crusader Esper, and while this won't happen for a long time (two of the legendary dragons are in Kefka's Tower), you don't want to have to come back here later on.
Ice Dragon
Don't be fooled by the Ice Dragon's tiny stature, this is a powerful Monster that likes to inflict Freeze with N. Cross. It also has a powerful attack and can use both Absolute 0 and Surge. If you have any way to gain resistance to Ice, this is a good time to use it. The Ice Dragon can counter any attack with a basic attack of its own, and may use Surge on death out of spite. Keep your HP up and hit the Ice Dragon hard with Fire, avoiding Ice. If your Characters are Frozen, you can use Fire to thaw them quickly (while dealing a minimum of damage to them). You can't run from this battle, and the Ice Dragon has 24,400 HP. Good luck!
Tritoch
You'll find Tritoch at the edge of the cliff where you chose between Mog and the Gold Hairpin in the World of Balance. Although Tritoch is a tri-Elemental Esper, it is currently encased in Ice and fights much more like an Ice-Elemental Monster. This battle will be similar to the one with the Ice Dragon. Fire is your best bet for offense, as Tritoch absorbs Ice and takes no damage from any other Element. It is virtually immune to physical damage (except from attacks that ignore Defense, but highly vulnerable to Magic. Despite its 30,000 HP, Tritoch should go down faster than the Ice Dragon.
In addition to using Cold Dust to Freeze a Character, Tritoch will attack with Ice 3 and Rasp. Rasp is also used to counter Magic, which could result in MP problems for your casters. Most special Commands, but not all attacks, may be countered with Cold Dust. These include SwdTech, Tools, Blitz, Lore, and Sketch. The chance of a counterattack is 1 in 3, so if those are your only source of offense for a given Character, it may be worth the risk. You can use Fire on your allies to thaw them when they are Frozen.
Defeating Tritoch nets you the Tritoch Esper and opens the path to Umaro's Cave. There are no save points ahead, and you can only leave by completing the area, so be sure you're ready before you jump in.