Several more times the creature approaches as you approach the prisoner, but each time it is rebuffed by fire and a vigilant guard. You arrive at the marked cavern to find Matheus chained to the wall, with chains which seem to grow out of rather than be attached to the wall. Just out of his reach is a key, and a torch on the wall presumably both provides light and keeps the serpent at bay.
The man looks up at the approach of the large party. "Who are you?" he asks weakly. "And what do you want of me? I've already told you, the Merchant's Guild in Nice will pay you a reasonable ransom for me. But beyond that, there's nothing else I can offer you."
Garcin takes off his helmet and shakes out his hair. "I'm Luke Skywalker, I'm here to rescue you!"
er, how about, "Come with me if you want to live."
.
.
.
.
Hey, if I can't amuse myself, who will amuse me?
Garcin will actually say, "Ransom's been paid. You wanna come with us, or stick around until what'shername changes her mind?" He'll inspect the key, manacles and lock mechanism. He doesn't trust anything so blatantly obvious.
Matheus did watch them lock the manacles with the same key.
"Oh, thank the Lord it's been paid," Matheus says with a deep sigh. "Please, get me out of here. That pirate queen is crazy." He looks down at the key. "That certainly looks like the key they locked me up with."
Garcin will glance at Antonius and then use the key to unlock Matheus. If he gets turned into a frog he expects a luxurious lily pad, plenty of flies and kisses from wide variety of princesses.
The manacles unlock, and you hear a hissing sound from the hallway outside.
As the manacles come off, Matheus rubs his wrists and expresses his thanks. "I don't know who you are. But thank you for freeing me." Then he hears the hiss. "What was that?"
Seeing the magi are occupied with the snake, "I am Garcin, lately of Malta, and these are my companions. Come quickly lest the native wildlife grow restless. Hopefully full introductions can wait until we are safely aboard? And perhaps also a business proposition, si?"
As you move to exit the prisoner's chamber a strong wind blows through the caves, extinguishing any torches. The serpent approaches from in front now, those who can see in darkness see it approaching before it enters the glow of the magical light.
Does the Serpent behave aggressive or threatening?
Yes, it is rearing back above the party, hissing and bearing its fangs.
"Flavius ! Distract that beast !"
Antonius will finish to draw (or not) a circle as large as he possibly can with a piece of chalk on the stone of the cave floor (or use a knife if the soil is made of dirt). He will shout to the sailors, Garcin and Mattheus:
"Whatever you do, don't leave the circle and DO NOT BREAK IT ! You WILL be more safe from this snake here then anywhere else in this cavern!"
Yes! 
Frederic will start with a “Pilum of fire” if possible to the head of the serpent. While doing this he will have his shield and sword ready (subtle magic).
If he gets the change he will also cast “Gift of the wolf´s legs” on himself and then running around the serpent attacking the tail with his sword.
Pilum:
coyotecode.net/roll/lookup.php?rollid=102056 = 7 (Penetration of 14)
Gift of the Wolfs legs:
coyotecode.net/roll/lookup.php?rollid=102057 =2 (He will lose a fatigue level)
Antonius: Who is this Flavius you always talking about?
Garcin and Pepe, frustrated at being denied their role in combat, ready bows and wait for a clear shot. They do not now if this counts as breaking the circle but figure Antonius will yell at them if it does.
Edit: Well, Pepe is happy as a clam being protected while readying a bow, but Garcin is livid.
The fire does not penetrate, and the serpent snaps back at you, with an attk total of 10, biting with long glistening fangs that are dripping what can only be presumed to be poison.
With a quick sidestep Frederic doges the attack. Then he starts a counterattack with his sword.
He swings his longs word against the serpent. This is not as precise and forceful as he hoped for.
Defence total 21
coyotecode.net/roll/lookup.php?rollid=102155 =9
Attack total 16
coyotecode.net/roll/lookup.php?rollid=102156 = 3
Antonius to the grogs: "Of course ! Good thinking ! Try to take it down with your bows !"
Garcin and Pepe will wait for a clear shot and let fly a volley. Garcin is clearly a swordsman, not an archer, but Pepe is right at home.
Garcin Attack Dex1 + Bows 1+1 + Bow attack advantage 3 + roll2 = 8
roll.coyotecode.net/lookup.php?rollid=102162
Pepe attack Dex 2 + Bows 5+2 + Bow attack advantage 3 + roll9 = 21
roll.coyotecode.net/lookup.php?rollid=102163
If we need damage rolls:
roll.coyotecode.net/lookup.php?rollid=102164
wow, both exploding!
not sure how to type this into the dice roller, so I just did the d10's. Both rolled a 10, exploding to 20, +6, so both do +26 damage!!!
(Dont forget that you can spend a fatigue level to double your ability.)