The high seas around Denmark. 5 years before the death of the King of Arendelle.

The seas of Denmark kick and roll tonight. Through the tossing waves sales a menacing and extravagantly decorated ship adorning a skull and crossed swords.
It's wood is painted black and it's decorations are a silvery white. "The Dead Man's Queen" is written on it's side.
A pirate ship. One of the most feared in history. It is packed with cutthroat pirates from every corner of the earth.
Assassins from the Far East. Ex-Slaves and Warriors from Africa. Norsemen. Germans. Spaniards. But most interestingly. Two young boys.
The men called them the Twins even though one was older. The captain took a liking to the younger.
A boy named Karthus who had lost his father to Barbarians.
Karthus was angry and willing to learn all he could from whoever would teach him.
His brother, Arthur on the other hand hated life on the ship and rarely spoke to anyone but the magical sword he kept with him.

The Captain's name was Grimme. It wasn't actually but his real name was unmentionable. The mentioning of which was punishable by drowning.
He had a hook for a hand, dreadlocked coal black beard and hair. One side of his head was shorn and on it he had the skull and swords tattooed. He wore a navy blue and black-trimmed coat that was as torn and rotten as his beard. He was covered in rings and piercings from all over the world and at his sides, his prized possessions. Two perfect cutlasses. Weighted and designed perfectly.
Each had a coin worth a kingdom pressed into the blade.
One was naturally designed to click into his hook hand.

Grimme was one of the last of the Golden age Pirates, and his time for retirement was coming. He trusted none of his crew to be his heir but when two scarred and angry little boys barely alive and ready to do anything for food turned up he saw an opportunity. He allowed them stay on the ship. Offering them no security other than food. They would survive of their own accord and would be welcome to do so so long as they proved useful.

Karthus possessed nothing special to Grimme but a black heart like his own willing to do anything to excel.
Whilst Arthur was a shy boy with the power to create and manipulate fire of his own accord.
Grimme used Arthur to burn sails and cripple ships, though the boy hated every second of it.
Karthus fought alongside the men and took his share of the bounty.
Arthur would accept nothing other than what he needed. He would always take books though, the men usually left them or let them burn with the ships they plundered. Arthur could touch nothing without burning it and would ask Karthus to bring him back books.
Karthus did so out of obligation but did not always remember. He occasionally read some of them himself.
Arthur loved to read. Loved to learn. His favorites were the adventure books Karthus usually brought him.

One evening Grimme asked for Karthus alone and he sparred with the boy, Who was beginning to be a match in fencing. Karthus gained many scars from Grimme, as well as many skills. Grimme made sure that the men left him alone and treated him as well as they would a man. After a rigorous battle the two sat down and drank.
"Karthus. Karthus, Karthus, Karthus." Grimme said. "I love that name. A sour name. Cold. Like being cut. Chills even an old sea dog like me. Eh boy!" The old pirate let out a dark laugh from his gut. "Better than my name... William Spitts... Tell no one you know that name boy. Maybe I won't kill you. William Spitts!" The pirate laughed again. This time stopping to let out a nasty cough. "I'm getting to be mighty old, Karthus. Too old for this life. A pirate can't sail forever. Not unless he plans to make a joke of himself. I do not. I need an heir, boy..."
"The crew would never serve a boy like myself, captain..." Said Karthus.
"Eh... Would you? You gotta sell it. Ya got a heart like me, boy. Nothin matters but you. That's how you live. You'll live long that way." He stopped to cough. "You love nothing. And you never should. Not gold. Not steel. Not women. Not the seas. Love is weakness and you need power."
"Power like my brother... He never has to try for anything. Just wants it to burn and it burns." Karthus said spitefully.
"Exactly. You need that!" The captain leans in. "And I think I've got you a way to do it."
Grimme pulls a cloth out from his pocket and reveals a black diamond-shaped gem with two jutting edges. "It's a Heart Stone. One of three made. The last one left. This here will suck that fire right out of his soul and funnel it into you."
"Why are you telling ME this... Surely you could take that power and become the greatest pirate who's ever lived... Why me..."
"Aye! You're a smart boy! But That's just it. I can't. You have to have that voodoo in your blood. Even just a little magical blood will do.
I had a lot of sons boy. From many women. They're weak though. Not fit for my throne! Id say you're the closest thing To a son I've ever had! You're a match with a sword and you're only 11! Haha!" He coughs.
"I would consider you to be the closest thing I can call a father, Captain..." The boy said nervously.
"Don't be scared, Karthus. Not of me... I can see a lot of potential in you. You got a wicked look in your eyes! It's settled! You'll be the captain! I'll start teaching you to captain this ship!"
"But the men.." Karthus begins.
"Will grovel. Yes. But they won't question me. And when we get you that power. They daren't breathe without your permission! It'll be glorious. That power is wasted on your brother! Think of the riches you could amass..." He said handing Karthus the stone. "Think of the power!"
"I do..." Karthus stares into the black Heart Stone. "Every waking moment."

Arthur sat reading quietly in his room. A metal box at the back of the ship. He said nothing. Gigas occasionally spoke when there was something to speak about but right now their was silence.
"Arthur." A child's voice is heard through the metal door.
"Hey Karthus!" Arthur responds and moves and sits against the door. "How are things?" The brothers daren't open it in case Arthur was to burn something.
Karthus sits against it's other side. "You know.. Not bad.." He scratches his head. He can feel Arthur's heat through the metal door.
"That's good I guess." Arthur plays with some fire in his hands.
"The captain asked me if I want to be first mate... He.. He says he's looking for an heir." He turns his head to project his voice not wanting to say it loud but also proud of it.
"...Karthus... We can't stay here..."
"Why not... Grimme has been good to us... Like a father." Karthus cups his hands, playing with his fingers.
"He's not your father, Karthus... You have a father..." Arthur sounds worried.
"No... I don't. He died Arthur... He's dead and he's gone." Karthus closes his eyes; remembering the day. "He died..."
"That doesn't mean he's not your father, Karthus..." Arthur turned his head towards the door.
"Well it sure as hell doesn't help me now..." Karthus slouched a little.
"Karthus... Grimme can't... He's not a good person. Not like dad..."
"Dad died! Arthur... And you let him... You ran away..."
"Karthus... He told us to.. He told me..." Arthur tries to fight tears in his eyes. He loses the control of the flame in his hands.
"To run? Yeah. Well look where we are now..." Karthus started to sound accusing. "Maybe things would be better Arthur if you hadn't ran... If you had just used the fire to save him..."
Arthur was silent.
"You don't deserve it, Arthur. You barely use it. You don't deserve to have that kind of power..." Karthus pulled the Heart Stone from his pocket and stared into it's blackness.
"Karthus... You can't. We can't stay here... These men are bad. And they're changing you."
"Everyone is bad, Arthur." Karthus' emerald eyes are cold and dark now.
"The people we hurt... They're innocent..." Arthur is crying. He watches helplessly as his fire burns some of his favorite books.
"Innocence is a matter of opinion. Grimme said that. We survive. No one can blame us for that. We call them innocent because they're too weak to protect themselves. And weak people don't deserve to have anything." Karthus sounded a lot less like Arthur's brother with every word. "You are weak, Arthur. You choose to be. I am choosing to be strong." He gets up and walks away.
"Karthus... Please..." Arthur sobs and fire builds around him. "Dad told us to stay together... We only have each other..."