Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
There's only one thing between Oliver and the hated Shadar: the Miasma Marshes. With Clarion in hand, Oliver can dispel the worst of the miasma and wade into Shadar's realm . . . but first he has to deal with the beasties of the Marshes. Will he make it through safely?
Enter the grounds of Perdida's palace with the Clarion in hand and approach the far door. You'll emerge on the Road to Ruin. Head north to the Miasma Marshes; once inside, Esther will use the Clarion to dispel the thick miasma and let everyone through. Shadar doesn't look ready to give up with a fight, however . . .
Make Your Way Through the Marshes
Save at the waystone and head north. You'll find a crossroads, and in the middle of them a purple puddle. Step in this and the whole group will be poisoned. There are entire paths composed of this venomous liquid, and you'll either have to tread very carefully—or use the spell Levitate—to get around them. (The poison aside, this area's pretty straightforward.)
Straight west of the split is a chest with an Espresso. Grab it and backtrack a short way to a northern path. Follow it northwest to find a blue chest containing a Set of Dead Man's Duds. With those grabbed, return to the split by the waystone and restore your energy.
Take the northeast path in the split and continue northeast. You'll find a dead end with an All-be-Gone. Follow the winding road through the marshes until you come to a three-way intersection; to the east is a chest with a Fruit Flan. Head west and all the way north to find a small curving area with a blue chest containing a Fiend's Fang.
Proceed further west. You'll hit another split when you begin heading north. Continue along the eastern bend to your north to find a Tonic. Return to the west and take a southern path to the southwest, where you'll find a purple chest containing a Suit of Night Armor. Expect a painful trip to this chest if you refuse to use Levitate.
Follow the path north to the rocky bluffs. In the wider area just south of the bluffs, you can shoot at a green chest to grab a Bottle of Sprite Dew. That's it for chests! Head north to the bluffs and follow them east. You'll find a waystone where you can save and heal. Continue north from here, to the swirling vortex of water that probably bodes ill for your party. Shadar's lackey Vileheart will taunt Oliver when he arrives, and the battle will be on!
This battle isn't too bad so long as you know to Defend and All-Out Defense in time. Vileheart bounces about the arena, occasionally delivering normal attacks but usually relying on his two specials to deal damage. The first, Vile Breath, is expected: it poisons everyone. Annoying, but easily blocked. More dangerous is the second, Viledriver, in which Vileheart slams the ground and does considerable damage to everyone near him. If you fail to block Viledriver, your character will be paralyzed for a short time. This can prove absolutely devastating if everyone is paralyzed at once. Pummel Vileheart when not Defending with whatever attacks and spells you like - holy elemental attacks such as Arrow of Light work particularly well.
Watch the cut scene in the aftermath of the battle. Needless to say, it does not go well for Oliver . . . and when it's over, you're playing as Esther, back in Perdida. Head to the entrance of Perdida. She'll discuss the problem a bit with Swaine, they'll vent their frustrations . . . and then Vileheart will show up, intent on finishing the job it started in the Marshes. Cut to a lengthy cut scene with Oliver.
The young lad has earned the Mornstar spell . . .
. . . and now you're in another battle with Vileheart! Yikes.
Your second battle with the beast is easier than the first. Vileheart has a lot less HP, it doesn't attack as often, and though it does have a few new attacks—Vilence cuts one person's HP in half, Viol Roar does damage and inflicts curse on everybody—it's nowhere near as troublesome. Beat it down with your normal attacks . . . or obliterate it with Oliver's newest spell, Mornstar. Mornstar will do as much damage as your average Superchaged spell, and you should have enough MP to whittle down Vileheart with Oliver alone.
With Vileheart gone and the truth finally out there, Oliver is ready to scale Shadar's castle, Nevermore, and bring him down for good. There are still questions to be answered—but they can wait until after the Dark Djinn is finally toppled. Onward to Nevermore!