Before going to Moria, you seek a contract with Undine.
Go to Undine's Cave
To go to Undine's cave, first go back to Venezzia. You can take a ship from Alvanista to Venezzia.
In town, talk to Elwyn's father upstairs in the trading company, and tell Doug where Elwyn went.
Go to the ship that took you to Demitel's island, and ask to go to the north island instead.
When you fill your food sack in Undine's cave, use Seafood to fill it, because you will often win Seafood from monsters in the cave.
Equip your party with with Aqua Mantles if you have them (use a Rune Bottle on a Mantle to turn it into a Flame Mantle, then use a Rune Bottle on the Flame Mantle to turn it into an Aqua Mantle). Equip Arche with the Mental Ring. Equip Klarth/Claus with the Aquamarine ring.
When you are ready, go east from the entrance, follow the path, cross the bridge and pull the lever. Then go up past the bridge, and go left and down the stairs.
Go down and get the treasure. The one on the top contains an Orange Gummy, and the one on the bottom contains a Spy Lens. Then go down the stairs at the bottom left.
In this room, get the treasure (Rune Bottle) and pull the lever, then go back up. Go straight up and go through the door on the left, then go to the right, down, and left to that switch that you pulled earlier, and flip it.
Go up past the bridge, and this time use the door to the north. Follow the path, opening the chest (Aqua Mantle) along the way.
In the next room, go up, past where you see the chest, then go left to find some stairs leading down. Go down, open the chest (Seafood), then go down the stairs below. Throw the switch and go back up.
Go up to the next room. Follow the path, past the save point, until you reach two levers. Pull the one that is farther to the north.
Go back the way you came, and you will see that the other pool is empty now. Go into that pool, get the treasure (Seafood), and go down the stairs. Pull the lever.
Undine is in the room with the save point and the two switches. You might want to go back to the ship to rest first. This will help you level up before the battle. You can also buy supplies from the guy in the upper left corner of the big room on the ship.
Before you fight Undine, equip Mint with a Mystical Rune so she can heal your party quickly, and leave her tactic at Value HP over TP. Equip Cless/Cress with a couple of Reverse Dolls. Set Arche and Klarth/Claus to cast many spells. Turn off Arche's Tractor Beam, Storm, and ice and water spells. Cless/Cress's Tiger Teeth and Lightning Bolt are recommended for this battle. For your formation, put Cless/Cress in the middle of the screen and put everyone else on the far left.
If you are ready to fight Undine, go up to the room with the save point, save the game, and pull the nearest switch.
Keep Cless/Cress far away from Undine and have him use his long range attack. Use Gummies to supplement Mint's healing.
When the battle is over, use a Rune Bottle on the ?Book that you won. It becomes another Rabionis. Then set Klarth/Claus and Arche's tactics to not cast any spells. Also, change your formation so that everyone is in the middle again.
To exit the cave, pull the lever to your left to drain the pool. When the path splits, go to the right, past the bridge, and down to that lever that you pulled earlier. Pull it now to drain the water that is blocking your path. Then go back up past the bridge, to the left, and down. Then follow the path south to leave the cave.
In Venezzia, tell Elwyn's father where his son went if you haven't already.
Elwyn, Elwyn's father, and Nancy will all be in the northeast house in Alvanista, in the middle of an argument. Help them. In the SNES version, that completes the side quest for Elwyn and Nancy. In other versions, you can go back to the trading company in Venezia and talk to Elwin to attend to Nancy and Elwin's wedding. You will get a reward for Mint if you do so.