Once there was a kingdom, a vast and beautiful kingdom where summer never ended and winter never came. And in this kingdom, there was a citadel on a mountaintop whose spires reached so high above the clouds that its residents were said to walk among the stars.

This citadel was the seat of a great king, known throughout all the world as a shrewd and powerful warrior. He had been the first to unite the Lands of Summer under one banner after seven generations of striving, and for this, he was called All-Father.

But even this was not the greatest of the king's deeds.

During his ascent, there had been dreams and whispers throughout the lands, telling of a monstrous king of terrible power arising in the Wilds of Winter, who would cast Summer's palace into ruins and drown the world in ice and snow. There had been only one way to avert this fate: a son of that palace must fell the king in battle.

And so the King of Summer had ridden against the King of Winter. He had turned Winter's armies to stone with a spell from his own lips, and slain their wicked king with a slash of his own blade. And for this, he was called Giant-Slayer.

Now, with his victory secured, the king was a bold hunter, a keen patron, and a generous host. Whenever a young knight rode off on his first campaign, sweet music would flow through the streets of his capital. Whenever the armies of Summer knew victory, treasures would pour into the citadel so all his courtiers could dress in fine silks from far-off lands and shining jewels snatched from the lairs of dragons. And whenever the king and his knights slew a monster, it would be brought to the banquet table with an apple in its mouth, and all those of noble birth would be called to share the feast.

Everything was shared: belly and haunch for the lords, liver and tongue for the ladies, and the bones thrown to the hounds. All but the heart, the sweetest part, which the king would keep for himself.

The king had a queen, fair, wise, and beloved, and she was as skilled in peacemaking as her husband was in waging war. She was a master of all emotions; she felt the passions and pains of others keenly, but never allowed them to cloud her mind. With a glance, she would know you, fears, flaws, and all; and with a few words, she could mend broken bonds and soothe your pain away. It was said that many of the king's best plans had had their beginnings in a whisper from his queen, or in her gentle, knowing smile.

She had not always been quite so beloved. When the king had first returned from sealing the peace in his new bride's homeland, there had been rumours that the young queen was mad. But in fact, the people simply did not understand her wisdom yet.

It was said that the queen spoke in tongues while she slept, and her nights were fractured by terrible dreams; but none yet knew that this was the gift of foresight, which could only be a blessing to a vast land with so many enemies.

It was said that the queen descended to the dungeons at strange hours of the night without so much as a dagger, and whispered softly through the bars of its cells; but they did not yet understand that a gentle word could pierce as deep as any blade.

It was said that the queen had been seen eating yew berries and filling her shoes with pins; but not until a lady-in-waiting found the queen brushing her fingertips through a candle flame as if it were water did anyone think to ask why. And when she did, the queen smiled and told her that it did not hurt, for her mother had taught her a secret art, that no pain may make her falter.

But soon enough, the queen had proven herself, and the people's faith in her was such that that none questioned it when she ordered that every looking-glass in the kingdom be destroyed. Yes, it seemed strange, but perhaps that only meant it was her wisest decision of all. The truth, they whispered in the decree's jagged, glittering wake, was that the kingdom had grown so beautiful that it could not look upon its own face and live.

In time, the queen gave the king two sons, and these sons were as different as night and day.

The first, Prince Thor, was strong and handsome, bold and charming, and all else that you could hope a future king to be, besides. In fact, he was better than any had dreamed, for he had been born with a gift:

When he first slipped into the world, howling for his mother, thunder rumbled over the citadel. And when the queen took him into her weary arms, he laughed, and wind sang through the eaves.

As he grew, he learnt to call up storms with a word, a breath, a thought. He was sure to be the greatest warrior the kingdom had every known.

And so every day he trained with sword and shield from morning till night, learning to defend his people from the enemies that waited beyond their gates. He had his father's honour and his mother's heart; his men were glad to follow him, and the people were certain that the future with him was bright.

But while the second, Prince Loki, was gifted with his father's cunning and his mother's laugh, he was also terribly weak.

He had a grave affliction, unseen but insidious, which confined him to the palace and left him reliant upon the queen's charms and potions to keep death at bay. And as long as he took his medicine and remained within the palace walls, he grew up pale and thin, but almost as hale as his brother.

But no matter how Loki argued, bargained and wheedled for a chance to step into the open air and wield a sword like his brother and father, just once, just to try, the king and queen refused. Even that (they reminded him, gently and invariably) could be his death.

So while the elder prince sharpened his sword, the younger took to the library, where he sharpened his wits and his tongue and his smile. The royal library was vast and taught many lost and secret arts, and in time Loki was training not only as a scholar or a politician, but as a promising sorcerer.

But, although Loki knew the palace that was his whole world nearly as well as its ghosts did, there was one place within its walls where he was never, ever allowed to go. Somewhere behind a heavy, locked door deep in the dark heart of the dungeons.

It was not the strongest door, not the most barred or guarded, but it was the only one Loki had never seen open.

Inside, he was told, was a monster.

Loki was never to approach it, nor even look upon it. The queen had made him promise.

When he was a boy, the door terrified him. Thoughts of the beast behind it would keep him awake long into the night and trouble his dreams as soon as he slipped into sleep – for what could be so terrible that even his father could not slay it, only bury it and hope that it would be forgotten?

But as Loki grew into a young man and his love of sorcery deepened, he became curious, and bedtime stories no longer satisfied him.

Yes, his father couldn't kill whatever was kept in there, but could Loki, with all his talents and his endless, idling time, not find a way? Did he not have a duty to try? It was likely the only chance he would ever have to slay a monster for his kingdom, and perhaps he would not feel so much lesser than his brother if he did. Perhaps his father would be proud.

He resolved that he had to know.

One evening, much like any other evening, Prince Thor went to visit his brother in his chambers at the top of the palace's tallest tower.

Loki's door was guarded, as it always was; Loki could never be allowed to stray more than a dozen paces from a chaperone, not with the ever-looming danger of his affliction. But no-one could think Thor a threat to his brother, and even if they had, he would still be the crown prince, so the guards bowed and granted him entry without a word.

The heavy lock clicked, the door swung open, and an icy draught ruffled Thor's hair; somehow, no matter how high they built the fire, Loki's chambers always stayed cold.

Inside, there was a spindly black cat, shredding the finely woven rug before the hearth and lashing his tail through the air like an impatient war-banner.

But as Thor crossed the threshold, the cat's ears pricked, his green eyes widened, and he sprung up, becoming a young man on his feet in a blink of an eye.

"Let's go out," Loki said, before Thor could even say good evening. His fingers drummed on his thighs, buzzing with contained excitement.

Loki meant only to leave his chambers, not the castle itself, but even so, Thor was suspicious. He knew that look. "Why?"

Loki raised one shoulder, smiling mysteriously. "To do something foolish."

Thor snorted. "Well, now I can't," he said, with a regret that was only half in jest. "You could have at least tried to trick me into it."

"Where's your famed sense of adventure?"

"Asleep after a hard day's work."

Thor was indeed weary from his training, aching and bruised with fresh blisters on his hands, and as much as he normally enjoyed Loki's whims, he didn't want to have to worry for his brother tonight. All he wanted was company for the brief hour remaining before supper.

He glanced over the chamber, over the fur-lined window-seat and intricate silver birdcage, and his eyes caught on Loki's writing desk.

The desk was in disarray, with ink pots, papers, and open books strewn haphazardly across it. Atop the pile sat an apple with a single bite taken. The basket behind had been overturned, a dozen more scarlet apples spilling like a dragon's rubies onto the floor.

"What have you been working on today?" Thor asked.

Loki slipped back in front of Thor, blocking his view. "Just the same as ever – pouring over manuscripts in solitude until I think my eyes might begin to bleed. All very clever, sadly. Nothing foolish at all."

Thor raised an eyebrow in scrutiny. "Bleeding? Is that a headache in the making?"

"It's a figure of speech. I feel fine. No – marvellous." Loki tipped his head, following Thor's gaze like a begging hound. "Come on," he pressed. "I never get to do anything exciting, not like you."

"Training isn't exciting. It's deadly serious and maddeningly repetitive."

"So you've never taken up your sword and pictured a tapestry of yourself slaying a dragon?"

"Never," Thor lied blithely. "I see my face often enough on the coins."
Thor sidestepped, pretending to study Loki's bookcase, and Loki followed, turning on his heel to block Thor's view again.

"Well, if that's true, then you need the change of scenery as much as I do. Think about it. Rules flaunted, tedium vanquished, and Father blissfully ignorant of it all…"

Thor rubbed a hand over his eyes, groaned internally, and finally cracked a long-suffering smile. "What's the plan?"

Loki folded his hands behind his back and smiled in return. His eyes, just as wide and green now as when he'd been a cat, glowed. "The door."

Thor was silent for a moment; it was as if a visor had closed over his face. "Absolutely not."

"Why not?" asked Loki, mock-innocent. "Don't you think you can handle it?" After a heartbeat, his brow knitted and he fell into a small, desperate smile. "Come on, brother. The beast's already been confined. I only want to see it."

"Do you think the beast will be content with seeing you?" Thor folded his arms. "It's the most dangerous creature in the entire kingdom. You will not be going anywhere near it."

"Compelling argument. Unfortunately for you, I will, because I already have the key and you can't stop me. What would Mother think if you let me die down there?"

Thor closed his eyes for several moments and allowed himself a quiet sigh.

Loki's smile widened.

Thor fixed him with a hard stare. "Do not let anyone see you."
Loki rolled his eyes, and then he was a magpie. He fluttered onto Thor's shoulder, feathers flashing blue-green and blue-black.

"Happy?" he laughed in Thor's ear.

Thor threw him an exasperated look. As smug as any knight, Loki preened.

And so the two princes set off to find the monster in the depths of the palace and look upon its hideous face.

The guards at Loki's door let Thor go without question, and the pair started down the stairs. Loki picked at his feathers and glanced this way and that in anticipation, while Thor in his wariness was quiet and still.

The shadows of evening were already long, and as the princes descended deeper and deeper down the spiral of the stairs, the light turned from gold, to red, to sapphire blue.

They passed ballrooms and banquet halls, where ladies played the harp while their lords sang to overflowing horns of mead. They passed cavernous royal kitchens, where each breath was heady with the scent of wood-smoke and honeyed meat. They wound through the cells of the dungeon, where the shadows were haunted by the rattle of chains and the sweet, sticky smell of blood. And all the way down, the guards were the same: upright and steadfast in their shining gold armour and wickedly sharp blades.

The last of the guards were stationed outside the double doors of the vault. Beyond that, they were alone.

The princes entered the vault warily, for they had never been there without their father before.

It was a shadowy and labyrinthine place: a great honeycomb of alcoves and arches, like the burrow of an ancient wyrm. And everywhere, secrets were piled up to the ceiling and spilling down the walls.

In the shadows glittered weapons of every kind, a few whose stories the princes had been told, and many more whose they had not. Books and manuscripts slumped in dusty stacks against every wall, almost enough to rival the library upstairs. One path was lined with standing stones, deep gouges running through all their carvings. In another passage, one wall was entirely devoted to the jewel-bright shells of dragon eggs, precisely cut and carved and mounted, while its opposite was lined with glass jars. Inside, unborn dragons chicks were suspended in viscous liquid like a plague of flies in amber.

And all around, wherever the princes looked, lay the gleaming, opalescent bones of unidentifiable creatures, scattered as if their owner had given up on ordering them, as if he had more than he could ever know what to do with. Once, Loki glimpsed a barrel which he was certain was filled to the brim with teeth.

But finally, at the very end of the great twisting hall, they found the door.

The princes halted when they saw it. Loki changed from magpie to young man once again, landing lightly on his feet.

He glanced at his elder brother, who had thrown out a hand in warning, as if he meant to shield Loki from the door with his own body.

Loki's heart hardened. He was tired of being treated like a child and made to look a coward.

He was as brave as any knight and twice as clever, and he would not let anything keep him from discovering what was behind this door. And soon enough, he vowed, he would return to this door without Thor, and slay the beast himself.

Loki snatched up a torch from a bracket on the wall, conjured the key to the door from thin air, and turned the lock.

With an echoing click and a breath of cold, dusty air, the door swung open.

Behind the door stood an empty, ordinary room.

Loki stepped inside. He swept his torch high and low along the walls, searching for a hiding-place or escape route to explain the creature's disappearance.

Thor entered after him and stepped into the centre of the room, putting himself between his brother and the far wall.

As he did so, he noticed a shadow there, as tall and broad as a man. Drawing closer, he saw that it was covered with a dark, heavy cloth.

He squared his shoulders, steeling himself to find the beast in chains underneath, and pulled back the cloth.

But instead of a beast, Thor found himself face-to-face with a looking-glass.

Thor was not old enough to remember when looking-glasses had been banished from the Lands of Summer. He had never seen such a thing before, and it seemed full of wonders.

Its smooth glass shone brighter and lovelier than any tapestry or portrait, and it cast his torchlight back to him, illuminating the whole room in gold. And most fascinating of all was his own face inside it, breathing, blinking, and frowning, imperfect and alive.

Thor had never seen his face so clearly in all his life. He had always been told he looked like his father, and he'd never known better than to believe it. Now, however, he wondered. Were his eyes not more like his mother's?

Thor laughed softly to himself in amazement. What harm could ever come from such a treasure?

But then Loki came to Thor's side, and Thor's heart stopped.

The Loki in the mirror was not Thor's brother.

It was a monster.

Yes, that creature shared Loki's silhouette like a shadow, and followed where he moved – but its skin was as blue as ice, its eyes were as red as blood, and at the tips of its fingers curved claws as black as night.

And Thor knew this face well, as every child in the kingdom did, from paintings and carvings, from tapestries and stained-glass windows, from bedtime stories and songs.

This was the face of the King of Winter.

Thor recoiled.

Loki fell to his knees.

Loki's torch slipped from his fingers, snuffing out the flame. His eyes were wide, but so were the monster's, their scarlet growing through that familiar expression like a bloodstain, or like the yawning maws of some terrible beast, as if just by looking at Loki, it might tear him apart.

Loki took a ragged breath, and then he reached out and touched one shaking hand against the glass.

At once, the blue seeped into his fingertips and spread up his arm, unfurling in fractal blades with a whisper of creaking ice as winter claimed him for its own.

Loki tore his hand away and screamed.

An icy wind rose from nowhere at the sound of his voice, tearing through the chamber as if raking the air with its teeth. Suddenly, the whole dungeon was as cold as the grave.

And where the second prince of Summer had been, there kneeled a monster, now blue from head to toe.

Thor's head spun, but it took no thought at all to draw his sword.

The monster scrambled back. Its hands were raised in a defensive and oddly imploring gesture, as if it were not predator, but prey. Its gleaming, tearful eyes seared into Thor's.

Thor felt sick. He stared, but he could not understand what he saw.

Where was his brother? How could this creature have stolen his shape and taken his place just by meeting his eyes?

And why did it look on him like this? If those monstrous eyes could weep, did it also have a heart like a person did, a heart that could feel fear and sorrow and pain – or was this only another cruel, cowardly trap?

Or – as much as it terrified him to think it – could this be his brother, this wild thing, this cornered animal?

Had they ever known one another at all?

The monster, staring back, was wondering too.

Before either could act, there was a thunder of footsteps, and the queen came running down the stairs. In a swirling cloud of cloaks, a pack of guards appeared close behind.

The queen, being wise, had ways of knowing when something terrible was happening in her palace, but often the warning came too late. The bitterness of the gift showed in her face: her brow creased, but her eyes clear; terrible sorrow, but no surprise.

"Loki, my dear," she said softly, "look at me."

The monster snapped his face up to her, eerily, desperately.

Loki? he thought, wringing clawed hands before his chest. Am I still he?

The queen stepped closer, ever-graceful, ever-gentle, even in such horrifying circumstances as this.

She reached down and brushed a tear from her younger son's scarlet eye. A sob shuddered through his chest, but even in this icy air, his breath could not be seen.

"Oh, my child," the queen murmured, cupping the monstrous face in her hand. "We meant only to protect you. You should have obeyed your promise."

For three heartbeats, the monster prince sat still as a statue before her, staring up helplessly through his tears as if bewitched. But then he closed his eyes and let out a shuddering breath.

And suddenly, Loki tore himself from the queen's grasp, lurched to his feet, and fled.

The guards parted like water around him, recoiling from the sight of his monstrous face, while the queen staggered and pressed a hand to her temple, looking more heartbroken than ever.

So it was Thor who led the chase, following the flitting blue-green hem of Loki's cloak up and around the breakneck spiral stairs, up past shocked and shaking bystanders, up through the echoes of screams.

Thor still did not know what he should say to this creature that had been his brother, but he knew that grave danger awaited if Thor could not catch him, the same danger that had caused their parents to keep him locked in the castle all these years – and he was certain that this was all his fault.

Thor sprinted all the way to the top of the tower. But when he reached Loki's bedroom, the door was thrown open, and the creature was nowhere to be seen.

Sweet summer wind blew through the wide, broken window. Over the forest, a cloud of birds had been startled into flight. The waters of the fjord frothed darkly far below.

A few moments later, the queen came to Thor's side and silently took his hand. He did not meet her eyes. His own burnt with tears.

The same truth was clear to them both, though neither could bear to say it: Loki had jumped.

They searched, of course, and the whole kingdom joined them.

The queen had guards and servants comb the citadel's every hiding-place. The king sent boatmen to discover what washed up against the rocky islands and cliffs of the fjord. Thor and his knights rode out into the forest, hounds baying as they called Loki's name.

But whispers of doubt soon began to wind through the court. Even though they had not yet found a body to bury, it was hard to believe that the prince still lived. If he had not broken his neck in the fall, then he must have succumbed to his affliction after so long beyond the walls of the palace.

If one day could have killed him, they muttered, then three were certain to.

And indeed, on the morning of the fourth day, the king called off the search.

Prince Thor was incredulous. "Loki is too fragile to be out there alone," he protested. "If there is any chance that we could still find him, any at all, we must continue!"

"He was fragile," the king said harshly. "And now he is dead. Because of you."

Thor stiffened at the sting of blame, but he pressed on. "We have no proof of his death. In your haste to assume him lost, you risk making us all his murderers!"

The king turned suddenly to meet Thor's eye, alarming the raven on his shoulder into flight. "Do you still not understand? He was lost from the moment you let him unlock that door!"

Thor looked to his mother, who twisted her hands and looked at the floor. For all her terrible visions, he had never seen her so full of sorrow.

As if he had taken a lance to the ribs, Thor found himself reeling and struggling for words.

"But – ought we not to recover his body? Can we not honour his memory, at very least?"

"Do you think he would wish to be remembered as the creature you let him become?"

Thor lowered his gaze, for he could not bear to answer. Tears gathered in his eyes.

The king sighed, and regarded his son with regret. "I thought you would be ready for the throne soon," he said quietly. "You knew your duties, and still you chose to indulge in these selfish, arrogant games…"

"Father, I am sorry," said Thor, his voice raw and strangled with grief. He swallowed. "I will accept whatever consequences you see fit."

The king flicked his eyes away in frustration. "Do you hope that I might banish or imprison you? No. You are my only heir; you cannot be released from your duties simply because you have failed them. But now that you have proven yourself unworthy of the throne, you will have to prove your redemption three times over before I relinquish it to you."

"How?"

"How far would you go, to protect this kingdom?"

"I would do anything."

The king hesitated, pinning Thor under his gaze. When he spoke, his voice was remote. "Your brother had the wisdom to slay the monster he found in the dungeons. Could you have done the same?"

Thor stared at his father, speechless.

Something deep inside him twisted uneasily – the king's words sounded wrong.

But who was Thor to question him? Surely the king would never have gone to such lengths to keep Loki safe if he truly meant him harm. And if what he was implying was true – if Loki had done such a dreadful thing to himself – then Thor could not imagine that there had been any other way.

No, Thor told himself harshly, it was his own defiance that had led to this disaster, his own reckless, foolish arrogance. This was his fault, and no-one else's.

And surely that was what his father meant to tell him: Loki, who had never so much as swung a sword, should not have been the one to die for the kingdom.

No. It should have been him.

"It is bitter work, becoming a king," said his father. "But the world is full of evil, and bloodshed is a sad necessity." His voice had softened, growing almost gentle. "You bear singular power, which must be with utter self-control. You have clung too long to your childish heart; you will only do more harm until you let go. When I know that you would truly do anything for the kingdom… only then will you be a worthy heir once again."

Thor sank onto one knee and bowed his head like a man condemned.

"Father, I promise you," he whispered, "I will not fail again."