"Or perhaps in Slytherin

You'll make your real friends,

Those cunning folks use any means

To achieve their ends"

Prologue

The lesson started off normal enough.

As usual, Elsa was early for class and took her seat along with a few classmates who had also decided to beat the rush of students that inevitably arrived at sound of the bell.

As she waited for her small group of friends to join her, Elsa took out her notes for today's Defense Against the Dark Arts lesson. They had been discussing defensive charms that robbed one's attacker of his voice. And after practicing with her fellow Slytherins the other night she had been able to successfully reduce Selina Moore's voice to a mere whisper.

Classes resumed and the room became a sea of blue and green, as Slytherins and Ravenclaws alike took their seats.

Once everyone had arrived Professor Lupin called for silence and informed them they would not need their books for today's lesson, only their wands. And asking that the desks be pushed against the wall he pulled a very large, very old looking wardrobe to the center of the room.

Elsa had long ago decided she liked Professor Lupin much better than that giggling buffoon Lockhart. Now her Slytherins were actually learning how to protect themselves, not which hair products would give you that extra shine. However, there was something about their new Defense Against the Dark Arts teacher that gave Elsa pause. It was his eyes; she couldn't quite decide what is was but she saw something familiar in them that made her fingers itch.

Elsa stood at the edge of the group and listened as Professor Lupin directed their attention to the now rattling wardrobe. As the the curious students craned their necks to get a better look Lupin began to describe the type of creature that lurked inside. The longer he talked the more panicked Elsa became.

"… shape-shifter… whatever you fear most…"

No. No. No. No. No.

When Elsa felt the tingling under her fingernails she instinctively pulled her gloves on tighter.

Conceal it, don't feel it.

"Hey Elsa are you okay? You look a little pale?" It was Tablitha Wilkins, one of the first few friends the blonde had made during her first year at Hogwarts.

Elsa blinked at her a few times to compose herself and swallowed. "I- I'm fine. Just a little nervous I guess."

"Yea me too," Tabitha said nodding and turning back to the front of the class. "But it's also kind of exciting don't you think? I wonder what it will turn into for me."

Elsa didn't answer her friend; she was too busy trying not to start hyperventilating. The blonde wasn't like the other students; she didn't need this boggart thing to tell her what she feared most. Elsa knew what she would see.

Anna. Her little sister, lying frozen on the floor. And this time there would be no healers at St. Mungo's to bring her back.

Even though it had been years since the incident, that night haunted Elsa every single day of her life since and fueled almost every one of her actions. It was why she almost never went home for holidays, it was why she wore gloves every waking moment, and it was even part of the reason she was in Slytherin house.

The moment Elsa had gotten her letter to Hogwarts she had been so relieved. Finally, a chance to escape the guilt that consumed her whenever she saw the unbearable hurt that flashed across Anna's face each time the blonde pushed her away. But she also knew that it wouldn't last. It was only a matter of time before Anna got her own letter, and Elsa knew without a doubt that her younger sibling would be sorted into Gryffindor. Even if you only knew Anna for a few moments you would know that was where she belonged. And it was with that in mind that Elsa asked - no - begged the sorting hat to put her in Slytherin. The young witch didn't care that it thought she could be well suited for Ravenclaw; it was well known that those from Slytherin and those from Gryffindor did not interact, and that was all the convincing she needed.

She would do whatever it took to stay away from her little sister, the person she loved most. To make certain the event of that night never happened again. And if that meant Anna thought she hated her, so be it.

The temperature in the room dropped ever so slightly. And so, while everyone else listened intently to Professor Lupin's lecture, Elsa was fighting to remain calm and quell the frost that threatened to break free of the seal her bewitched gloves provided.

It's okay, no one even knows Anna. Come to think of it, I'm not sure I even told anyone I have a sister. And even if they guess, they will just assume I'm afraid of loosing a family member not that I'm afraid I'll be the one that kills her.

The room grew colder. One of the girls rubbed her arms.

The spell must be wearing off, I need to go see Dumbledore. Conceal, don't feel.

Slowly, Elsa reigned herself in and felt the warmth seep back into the room. Still chanting her personal mantra, she felt her breathing even out and no longer felt the need to bolt out of the room. Looking around quickly she was relieved to see that no one had to noticed how close she had come to coming undone.

Her heart still pounding furiously in her chest, Elsa tried to focus on whatever Professor Lupin was saying about the different theories regarding the true form of a boggart and how to properly disarm it.

After a quick lesson on the annunciation of the defensive incantation and proper wand movement, Professor Lupin asked the students to form a line in front of the large piece of furniture.

The class surged forward, everyone vying to get to the front; curious to see what it was they were most afraid of. Somehow in all that chaos, Elsa found herself at the center of the long line, sandwiched between her friend Tabitha and some Ravenclaw that looked about as thrilled about the idea of meeting his worst fear as she did.

The first student to step forward was a boy from her own house whom she had tutored a few times in Transfiguration. At Professor Lupin's instruction he stood at the ready, wand raised and staring straight at the handle that slowly began to turn. The door of the wardrobe flew open and out ran a dog that Elsa could have sworn was the size of small bear. It was barking and snarling at the boy who was now backing away in terror with each flash of long, white teeth.

It was then that Elsa finally understood where he'd gotten the scar on his forearm.

For a second the witch forgot her fear as a wave of protectiveness washed over her. Elsa was very defensive of the other students in her house; it was one of the reasons she was so well known throughout the school. They may call her the Ice Queen, but they also knew that if you were going to poke fun at a Slytherin you'd better be willing to face Elsa as well. She was about run forward when Lupin's voice stopped her.

"Come on now William, concentrate. Be stronger than the fear, don't let it consume you. Turn it back in on itself. Laugh in it's face," he said fiercely.

The Slytherin boy nodded and raised his wand shakily. "Riddikulus!" he shouted. There was a small pop and the large dog became a tiny, shaking thing in a pink sweater. The class howled with laughter and William smiled.

Elsa radiated pride as she heard the Professor yell, "Excellent! Excellent William! Next!" The line shuffled forward and another student took the boy's place.

Up next was girl in Ravenclaw who's worst fear was a man wearing a mask and covered in blood charging at her with a knife. Then another Ravenclaw for whom the boggart changed into a swarm of stinging bugs that flew around her face buzzing loudly and threatening to bite her.

When a girl from Slytherin broke down at the sight of her parents screaming at her for being a disappointment Elsa became hopeful that Lupin would end the lesson early. He didn't. And with each student that moved forward the more terrified she became at the thought of seeing her sister's dead body in front of her. How on earth could she make that funny?

I can't do this. I won't be able to control it.

But she thought about what Professor Lupin had said to William. About not letting the emotions consume him, being stronger than the fear, and before she could react, Elsa was at the front of the line.

She hadn't even seen it change shape. One second it was a toy snake, the next it was…

Elsa turned to her Professor who wore an expression of equal parts confusion and concern. "I- I don't understand. I'm not afraid of mirrors," she said, lowering her wand.

"I'm not a mirror Elsa."

The girl spun around so fast she almost collided with her reflection. No, not reflection.

Her.

Time seemed to grind to a halt as Elsa stared in horror at her twin. Blood pounded in her ears and her breaths came in short gasps as her entire body began to tremble violently. Cerulean eyes stared back at her and a small smirk pulled at her- it's lips, as if it knew exactly what Elsa had done, what she was capable of.

She felt the world tip dangerously to one side as the reality of what was happening set in; her worst fear wasn't killing Anna, the thing that she feared most… was herself.

Behind her people began to whisper, while other's – maybe her friends – hushed them. Although, they might not want to be her friends after this.

"She is afraid of herself?"

"What did she do? Is she like a criminal or something?"

"Who is afraid of themselves? She must have done something horrible."

Then she heard someone scoff loudly at the back of the room. "What the hell did you do Arendelle? Kill someone?"

She went still. Her gloves grew stiff with frost.

"Shut up Draco!" Snapped a voice.

"Quiet!" Yelled another - Lupin possibly - she couldn't hear much over her own thundering pulse. "Elsa?" The voice called softly. Definitely the Professor.

He reached out to Elsa, hoping to capture her attention long enough to get between her and the boggart. However, just as he was about to touch her shoulder - which seemed to be radiating cold air - the boggart mouthed something to the girl. He didn't see what it was but it seemed enough to push Elsa over the edge. She tore out of the room so fast mumbling something over and over that he barely had time to react.

"Elsa!" he called, but she was gone.

xxxx

Elsa ran as fast as her legs would allow, lungs burning and tears streaming down her face. She only came to a stop when she'd reached a large hill overlooking the castle. No one ever came out there since it was so far away from the school, making it the only place Elsa felt safe enough to practice her wandless magic. She'd spend hours making small flurries and constructing objects from her ice.

But today was not a day of freedom, today was a day of hiding. Just like she always did. She was coward; too afraid to face her classmates, too ashamed to look her own sister in the eye, and too stupid to think she could run from it all.

Lying crumpled at the base of a large oak tree Elsa hugged herself tightly and wept. She didn't bother holding back her ice, letting it roam free as frost climbed the old oak's trunk like a crackling, hissing vine. Arctic winds howled around the young girl as sharp snowflakes bit at her skin, but she ignored all of it.

All she could do was picture the boggart version of herself ginning at her with such cruelty in it's eyes it made Elsa want to fall through a hole in the floor just to escape it. How it had mouthed the one thing it knew would shatter all of Elsa's carefully placed defenses, tearing them down and stripping her bare. The scene replayed over and over again until it was seared into her mind like a brand. She watched an endless loop of her own lips caressing that soundless word like a newfound lover until it was all she knew, all she cared about.

Monster.

Authors Notes:

Hello everyone and welcome to my new Harry Potter x Frozen fan fic. This particular story was inspired by the AU referred to as exolvo, born from the minds of counterpunches, arendellesfirstwinter, and ultranos on tumblr. If you would like to see all the headcanons and other fan made works simply search exolvo on tumblr. My story is very loosely based on this AU but only in a few ways.

The prologue takes place in Elsa's third year as she is in the same year as the golden trio. The remainder of the story will take place during the Order of the Pheonix - Elsa's fifth year and Anna's second.

Any and all question will be answered promptly on my tumblr account hogwarts-is-frozen, and as always enjoy and review, review, review :)