Hint: The army you need to get back is a "dead" army.

Hint: How do you meet the dead?

SPOILER: Kill yourself. Drowning is a convenient way! (Side note: I agree with Ran that this was a pretty strange quest requirement by NWC. This is one of the few very "forced" quest actions in MM9 that does not live up to the caliber of the rest of the RPG.)

But the Gatekeeper won't let you in. You need a "Writ of Fate" to enter. So he sends you to the Dark Passage.

This is a fun dungeon. Clever design. Classic RPG!

Entry: Be sure to note the parchment about the ghouls, skeletons, liches, etc. It has an important set of clues for the bridge puzzle.
At "A" is a big blue figure (A Reptilian Terror). Note that if you touch it, "something" seems to happen, but it is not clear just what. (For MM8 players -- remember switch "A" in the top of the flooded Minotaur lair?)
Head down the ladder to the bridge segments. At the far end is a crystal behind a locked gate that you cannot open. Even if you trick the system (for example, by raising the final bridge and running over to click the gate) it will not open until all bridge segments are in the up position. Pressing the various statues on the bridge raises and lowers segments of the bridge in no apparent order. When they are all up, the gate opens. Touch the crystal and you are transported into the next section. Hint 1: The scrap of paper you found when you first came in Hint : It's simpler than you think. Ignore all the details in the note (like "X" doesn't obey "Y", etc.) Hint 3: Notice the sequence of capitalized names. SPOILER: Touch the statues in the same order as the note. You will have to swim a couple times. If you goof up, climb the ladder and touch the big statue to reset the bridges to their starting point.
Fight through the spectres and enter the banshee maze. This one is also easier than it appears. Look around and find the statue (of Kira?) that unlocks the gate to the next crystal. Hint: there is a hard to spot ramp leading up on the right as you enter the room.
Other than the fights and a hidden door, there is no puzzle in the Colloidal Forest. Just touch the crystal.
It's a long fight down the next hall. When you can see the water, jump in, take care of sea monster #1 (firing arrows from above is almost no fun), swim through the tunnel to the next room, and climb up on the island. (Note, Judy reports you cannot swim through the tunnel until the 1st sea monster is dead.) The next crystal is covered by a cage. Dive in, and when you have eliminated sea monster #2, the cage is gone and you can move on.
The next area starts tough!! 6-8 Reptilian and Desert Terrors. In each corner "C" is a column you can get behind that lets you take them on one at a time and easily.
Upon entry to the mirror puzzle, I briefly saw Njem in the distance. He warped out before I entered, though.
The Mirror maze: If you study the area, you will discover four mirrors (1,2,3,4 on the map), a crystal at "5", and a pedestal at "X" that shoots a beam towards mirror #1 when you touch it. There is a locked door behind the crystal. Touching the crystal does nothing. Hint1: Wonder what the mirrors and the laser beam do? Hint2: If you touch a mirror, it rotates slightly. You can "aim" them. SPOILER: Rotate each mirror so that the beam from "X" shoots to 1, 2, 3, 4, then hits the crystal. When you are successful, the doors on the sides of the area open and 6-7 Desert and Reptilian Terrors attack. If you run to the SE corner, you can back up a gully that lets only 2 or 3 attack at a time.
Before you go through the locked door, save your game! Cast torchlight and wizard eye! There's a trap on your left, and a few invisible spectres. Head through the secret wall at the far end.
The ramp puzzle is fun. Clever RPG. Hint -- notice the pattern of lines and the switch on the front wall. Note the four spots on the floor. SPOILER: Turn the switch, then step on the four spots in the same order as the wall lines (3-1-2-4). On "4", it will rise and lift you to where you can jump onto a ramp.
Finally ... the end of the Dark Passage. Except now you find that getting the Writ is not that simple. First you must get the Capstone of Order.

Head to the Verhoffin Ruins again, step on the pentagram in the correct order, and enter the room. Place the capstone on the pedestal at the far end of the room, and then make sure you pick up the Writ of Fate off the table, behind you, before you leave! If you die without it in your inventory you will have to restore from a saved game other than the autosave. (See Praetor's bug in the "Scripting Errors" section of the bugs summary.

Now, with the Writ safely in your pocket, head back to Arslegard again via the usual way. Glug ... glug ... glug.

(See the Isle of Ashes Part 2 walkthrough for map details.)