“If your sword hadn’t chopped through that sea rover’s spear,” said Ashurbaal, captain of the Duke’s hired mercenaries, “it’d be me feeding the beach crabs tonight. Not many men have Ashurbaal the Red owing them.” The brown-skinned southerner bent his head close to Cory’s. “You’ve a knack for weapons, lad. And the sense to use advice. Come south. We’ll slip you aboard with us and free you from this cursed land.”
Both men raised their eyes to the promontory where the Duke’s castle was building. Nothing showed yet. Its dungeons lay open to the sky, but an eldritch blue light flickered above them. At the sight of that ghostly light the mercenary captain’s hand fumbled at the neck of his embroidered tunic. He brought out an amulet of human bone and red wool twist which he stroked and

quote:

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“If your sword hadn’t chopped through that sea rover’s spear,” said Ashurbaal, captain of the Duke’s hired mercenaries, “it’d be me feeding the beach crabs tonight. Not many men have Ashurbaal the Red owing them.”

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With that long, high-information tag in the middle of a sentence of dialogue,I lost track of what was being said. I'd recommend either moving the tag to the end of the sentence or dropping 'Ashurbaal' from the tag since we learn his name a few seconds later in the dialogue. I also believe that mercenaries are obviously hired, so you can cut the tag to "said the captain of the Duke's mercenaries."

quote:

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The brown-skinned southerner bent his head close to Cory’s. “You’ve a knack for weapons, lad. And the sense to use advice. Come south. We’ll slip you aboard with us and free you from this cursed land.”

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I wasn't initially sure if the brown-skinned southerner was another description for Ashurbaal or if he was a new character. When he says "We'll," I'm wondering why the merc captain is running off. Is he leaving? Or is the southerner a sailor who is referring to his fellow ship-mates in that sentence?

quote:

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Both men raised their eyes to the promontory where the Duke’s castle was building. Nothing showed yet. Its dungeons lay open to the sky, but an eldritch blue light flickered above them. At the sight of that ghostly light the mercenary captain’s hand fumbled at the neck of his embroidered tunic. He brought out an amulet of human bone and red wool twist which he stroked and

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If nothing showed yet, then how come they can see the dungeons? Also, the way you have this currently written, you have the castle doing the building. Perhaps you mean "where the Duke's castle was being built" or the like? I also don't understand how the dungeons being open to the sky is connected to the freaky light. As in, why do you use "but" as the connecting word?

quote:

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“If your sword hadn’t chopped through that sea rover’s spear,” said Ashurbaal, [Do you need this so soon?-->] captain of the Duke’s hired mercenaries, “it’d be me feeding the beach crabs tonight. Not many men have Ashurbaal the Red owing them.” [Does this The brown-skinned southerner sound better than: Ashurbaal] bent his head close to Cory’s.[Who's Cory? Maybe you should have him reply, or acknowledge Ashurbaal's comment in some way.] “You’ve a knack for weapons, lad. And the sense to use advice. Come south. We’ll slip you aboard with us and free you from this cursed land.”[What cursed land? Why is it cursed? I thought that the previous battle was AT SEA.]
Both men raised their eyes to the promontory where the Duke’s castle was building.[The Duke's Castle was BUILDING? Why are they at the Duke's Unfinished castle? Does Ashurbaal not like being CAPTAIN of the Duke's imported warriors?] Nothing showed yet.[<--Do you need this?] Its dungeons lay open to the sky, but an eldritch blue light flickered above them.[If the blue light is important, you may want to start more focused on it.] At the sight of that ghostly light the mercenary captain’s hand fumbled at the neck of his embroidered tunic. He brought out an amulet of human bone and red wool twist which he stroked and...

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I agree with the comments so far.

Slow down your story a bit. In those 13 lines there are 6 events:

1) the sea battle;
2) Captain thanking Cory (?) for saving his life;
3) plans to escape something;
4) the building of the castle;
5) the blie lightening; and
6) introduction of the amulet.

Pick one main event and tell me about that in the the 13 lines.

You might cut the first paragraph and start with the men seeing the lightening in the castle and reacting to it. The other details can probably wait.

Both men raised their eyes to the promontory where the Duke’s castle was building. [DO YOU MEAN "BEING BUILT?"] Nothing showed yet. ...

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So the big issue I see here is for us to know who the characters are. What might help

* Never denote one solely by his description. If the Duke is the brown-skinned southerner, you might call him "the Duke, a brown-skinned southerner..."

First: why do you chop the first dialogue line to insert the tag? The phrase is convoluted enough without confusing it further by cutting it in half. I find the dialogue a little verbose. How about something like this:
"If you hadn't chopped through the sea-rover's spear, I'd be feeding the crabs tonight." I've cut a few words which I don't think were needed (why would I care if it's a sword or a cudgel that cut the sea-thingy, we'll find out soon enough, and "beach crabs" and crabs are the same, "it'd be me" is complex, simplify into "I'd be"...)
I assume "the brown skin southerner" is the speaker, but I'm not sure. I'm guessing he's talking to the MC, but I don't know it for sure. I need a POV character and a clear protagonist.

Cory is the pov character, not Ashurbaal.

Not a pirate story. The next few paragraphs would set the plot. It's about that eldritch blue light hovering over the dungeon-in-progress. That's where Cory's headed in the scene after next.

I just got this picture in my mind about a castle with this horrific dungeon where an evil guy tortures people and all and how once upon a time that place was just being built. All those cells lay open to the sky. Evil came there. That's what the story's about. Evil arriving. Expected evil. As I intimated, it is a dark fantasy.

There are a slew of innecessary and sometimes redundant words clogging your narative. Too much detail will slow things down; try cutting some:

If you hadn’t chopped through that sea rover’s spear, it’d be me feeding the beach crabs tonight. Not many men have Ashurbaal the Red owing them.” The brown-skinned captain of the Duke’s mercenaries bent his head close to Cory’s. “You’ve a knack for weapons, lad. And the sense to use advice. Come south. We’ll slip you aboard with us and free you from this cursed land.”
Both men looked to the promontory where the Duke’s castle was being built. Its dungeons lay open to the sky, but an eldritch blue light flickered above them. At the sight of that ghostly light the captain fumbled at the neck of his tunic and brought out an amulet of human bone and red wool twist.