That's the passage to Preamarl

Welcome to Preamarl

Go see the Queen

That's the location of the Dual Blade

Bad news

The Queen will try to save her daughter alone. Go after her.

Go to the location the Queen gave you. That's where the princess was swiming. After navigating the canals a litte, you'll end up here

This fight is very easy. Most of the time, the ship will summon some friends : Deadly Swords and Hades Skulls. Fry inflicts major damage. Be sure to use other holy weapons.

This is not the end of your problems. The mermaids will get sucked in and you'll hear Daos. He'll form a path for you to cross. Take the warp and you'll end up in this room. Even though its massively screwed up (yet another Natsume problem), you can still go around it. You'll eventually get here.

Daos will explain what the Dual Blade means and why the Sinistrals are present.

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To show his power, he'll warp you to Chaed

And from Doom Island, he'll destroy the town.

Daos will taunt Maxim by asking him to join him on Doom Island.

Since you can't fly, go see Lexis.

He'll share his idea with you and tell you you're gonna have to steal a more powerful engine than your ship has. Remember Gratze from Dankirk ? They tried to steal the Ruby Icon.

Since they're out to take over the world and they have better technology, then its alright for you to steal despite what Artea said to Milka when she took the submarine's plans.

Go here to access the passage to Gratze

You'll notice the bridge was broken. This saves you the gruelingly boring trip through the North Cave.

Unfortunately, you're caught as you exit the passage from the cave.