The Dead Sea
Travel to the Dead Sea, the massive, hurricane-like cloud bank on the east side of the map. Use the Fiddler Crab you received from the Sage of Marbule near the south of the landmass, where rushing water is visible and the name of the location appears. This will create a pathway to Death's Door...
... which is an apt name, since you can't cross here alive. A legendary, evil sword known as the Masamune is blocking your path, and you can't get past it as you are. You'll need another, equally-legendary sword to counteract the Masamune, which means finding the Einlanzer. Radius will explain the history of the two swords, and point you to Hermit's Hideaway. Once there Radius will tell you more about Garai, the former wielder of the Einlanzer, and fetch the Garai Keepsake. This will allow you to fully navigate the Isle of the Damned, your next destination.
Isle of the Damned
A rather nasty place that you may have visited before - but never fully explored - the Isle of the Damned will tax your abilities with some tough enemies, the most difficult of which is its final boss. You'll run into the following monsters while exploring the Isle of the Damned:
- Will o' Wisp - Red Innate. Little phantasmal lanterns that rely on Red Innate Elements. Smack them down with Blue Innate Elements of your own.
- Dead Beat - Black Innate. You met these little guys on the S.S. Invincible. They're still not a big deal.
- Dead Head - Black Innate. A boss? Returning as an enemy? Dead Heads here aren't as annoying as the one on the S.S. Invincible, but they're still tough. Hell-Bound and Hell Soul can put a dent in your team quickly if your luck is poor, so take these things out first.
- Airframe - Black Innate. Skeletal bird demons that can divebomb you and do some serious damage. Wipe them out quickly with White Innate Elements.
Go north from the entrance until you're on higher ground, then follow the path clockwise, to the west. There's a doorway blocked by a Will o' Wisp that you can't open until you beat the monster. Once you do more of them will spawn on the first screen of the Isle of the Damned from this point on.
(While wandering through this first screen you'll see two ominous, glowing barriers, one in the southeast, one in the northwest. These two spots are open in Another World, but can't be unlocked here. Don't worry about 'em.)
The next screen brings you to a series of paths that will loop around on each other until you learn which ones to take. Get into the ribcage that runs from north to south through this area and follow it to the south. You'll appear from a cave on the left side of the screen. You can use this to reach a nearby mirror door. Use the Garai Keepsake in front of the doorway to grant yourself access.
The next area has a small spot in the upper-right corner of the screen that constantly spawns Will 'o Wisps, creating a new one when the previous is defeated. If you step too close to their spawn point you can be trapped in a loop of battles, so avoid this spot. The Will 'o Wisps will chase you around the area.
If you walk north and counterclockwise around the west side of this area you'll find a suit of armor. Defeat a Will 'o Wisp beside the armor it will explode and awaken the armor, putting you in a battle with a Wight Knight, a powerful Black Innate enemy that uses buffs and debuffs to make itself a potent force with its sword. If's highly advisable for you to defeat the Wight Knight with Sprigg, as its one of her best transformations - and this is your only chance to get it. You'll receive a Golden Tiara after defeating the Wight Knight.
Through the central door in this area you'll find a grave marker, and you'll learn more of what Radius did to his friend, Garai. Still vengeful, Garai will rise up from the grave and attack your party.
Garai
It has been a while since you've fought such a potent warrior. A White Innate, Garai uses punishing physical attacks and powerful Techs to maul your party. Triple Cut will do a decent amount of damage and can Fatigue a character, while Will-Breaker inflicts nigh-lethal amounts of damage on a single target. It's not exactly a vast repertoire, but given room to breathe Garai can crush your party, one-at-a-time.
If you're having trouble with keeping your Black Innate characters alive - namely Lynx - then you should either have multiple Revives available or just change Garai's Element altogether using one of the six Turn Color Elements. Will-Breaker will still do a lot of damage, but it shouldn't be a OHKO if Garai isn't White Innate. You can then target his new weakness. Using Lynx and Harle to pummel a White Innate Garai is also an option if you think you can take him down quickly, though be ready to Revive one if they take an unlucky hit from Will-Breaker. Stock up everyone with powerful healing Elements, and use them often so everybody's health can withstand a flurry of blows. This will also prevent the Field Effect from becoming all White.
You'll earn a Star Level, 1,358 g, and a Dragoon's Honor for defeating Garai. In the aftermath of the battle you'll also receive the Einlanzer, which, alas, you can't use as an actual weapon right now. (Though you can eventually!) That's all for the Isle of the Damned, so make your way back to the entrance and set sail for the Dead Sea.