RENAMED. Previously Merlin and the Secrets of Hogwarts.

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We are one year behind Harry in Chamber of Secrets timeline.

This is a multi-crossover but you only need to be vaguely familiar with Harry Potter and/or Merlin in order to enjoy it.

Regarding Jack and Elsa, they have a different backstory which will slowly be revealed, so even if you're not familiar with Rise of the Guardians or Frozen, you will not miss out.

I'm a research freak so there are fandom details everywhere but I also expand the magic of the HP universe, taking from Celtic mythology. We get up to mischief with Peeves and exact long-overdue karma on Snape. We explore a fairy colony, Merlin's home, and the Crystal Cave. We get into Old Religion - the beliefs and the magic - expanded past Merlin canon. There's fluff and there's serious stuff. I give you EVERYTHING!

UPDATE: The draft of this story is finished - only needs edits, so yes, there will be a conclusion! It's a long one but it only gets better the further you read.

My other story The Well of Youth is a backstory to this one. It's not required reading, but feel free to check it out.

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Rating T just to be safe. There might be some darker themes explored down the road.

I do not own characters or plots from Harry Potter books or Merlin or Rise of the Guardians or Frozen or… I guess that's it for copyright stuff. ;) Enjoy!

Chapter 1: Snowballs and hot dogs

1992, September 1st

Albus Dumbledore looked out the window overlooking the school grounds. Hogwarts Express was approaching in the distance. The steam followed the engine faithfully, making it look like a toy train set from this far away. Soon, a new student was going to come out of that train - a student more capable than even him - the legendary Merlin, an ancient immortal sorcerer who represented a precaution, but was his protection going to be enough?

The previous school year rang around his mind like an echo in an empty basin. Voldemort got in his school. Despite the wards and protections in place, despite a team of very capable wizards and witches ready to protect the students, he had found a way in. Aided by a Hogwarts teacher, he walked among them unnoticed and fooled them all, even Albus.

"My teacher!" Albus accused his reflection in the window. How did he fall for it that easily?

Voldemort was brought into the school by one of the teachers Albus had personally picked. That was the biggest slap in his face as if the Dark Lord was taunting him. Would this school year be any better? Could he protect Harry Potter so the boy could fulfill his destiny?

What Albus didn't know was that the train contained more than just the usual: students, faculty and their belongings. Right beside Merlin sat two unknowns, two magical beings pretending to be ordinary students, two wild cards which could sway the events of the coming year one way or another - magical twins whose destiny was intertwined with the immortal sorcerer.

And unbeknownst to them all, the train contained a sinister object. It was a part of a plan that was put into motion centuries before, realized but interrupted a few decades ago, and ready to be resumed now, in the hands of an innocent.

Albus had no idea how much danger his school was in.

1987

Elsa ran as fast as her little legs could carry her. Earlier, Jack had fallen through an opening in the ground and got separated from her. He yelled from the hole that they would meet at the bottom and she hoped she could find her way alone.

She couldn't count how many times they had attempted to escape the wretched tunnels with no luck but they had to keep trying. They had to have hope that there was something better outside than the life they had. Hope was the only thing that kept them going.

The darkness was becoming thicker and she kept tripping or running into jagged walls. She fell down again painfully.

"I won't cry," she promised herself. "I won't cry."

She sat up and rubbed the dirt off her knees which were now bruised and bloody. She was hurt, lost and had no idea how to get to her brother.

She hated the darkness. It filled all of her nightmares and surrounded her during waking hours. Now, it seemed to loom over her, suffocating her, sucking up her hopes and dreams.

The twins dreamt of the wonders of the world outside but could never get there. Every attempt at escaping had always been a failure. The hags made sure of that. Their mother made sure of that. Elsa started to wonder if the exit was magically concealed and they had in fact passed by it many times, not realizing what it was.

She was sure now that they weren't going to be able to get out today. The hags would ask them where they were again. 'We got lost while playing' was their usual excuse. Did the hags suspect that each of those incidents was an escape attempt? There was no way to confirm, but the number of previous attempts might be working in their favor. She hoped that the hags only thought that the two of them had a horrible sense of direction.

But now she had a new problem. She was lost and couldn't even get back the same way she got there. If no one found her, she could starve alone in this gloomy corridor. Would she waste away in darkness?

She wrapped her arms around her bent legs and closed her eyes.

"I won't cry. Jack will find me," she assured herself. "I won't cry."

All she could do was hope that her brother would find her but how? He didn't know this tunnel any better than she did. It was their first time here. How would he know where to go?

Panic was ready to sink its claws into her and she held it at bay only with the thought of her brother. She tried to distract herself by focusing on the loud thump of her heart. She pictured her ear pressing against his chest and wished it was his heartbeat she heard.

'Jack, find me,' she thought a prayer as the first tears fell on her cheeks.

She found comfort in imagining that he was with her and her hope grew to magical proportions. She always felt better when he was close. He made her feel safe.

In a few minutes, she heard quiet footsteps and looked toward the sound, afraid of what nightmare might come her way. Was it a hag? Was it something worse? Mother always warned them that dark creatures loomed in these tunnels.

She jumped up seeing something approach but then sobbed happy tears, recognizing who it was. Jack's snow-white hair stood out in the darkness, chasing away her fears.

"Elsa!" her brother ran up to her and pulled her into a bone-crushing hug. "I heard you," he said while squeezing her. "I heard you in my heart."

1992, July 1st (present day)

Minerva McGonagall breathed in summer afternoon air laced with smells that spoke of freshly mowed lawn and summer picnics. She finished her errands in the city and now had a little time for herself. The school was on summer break and she enjoyed her time off. She loved teaching and devoted her life to it but even she had to recharge her energy. A walk through her favorite London park was exactly what she needed.

She wanted to take her usual route around the lake where she could watch the ducks but found her path blocked by a dog-walker who had nine dogs of various breeds and sizes with her. That was excessive. Who could control this many beasts?

Minerva did not want to encounter them, so she took a detour - a little pathway between the trees.

She was reflecting on the impracticality of traditional wizard attire in the summer when something dropped right next to her feet. She looked down to inspect the splatter of white on the ground and gasped when she realized that it was snow. Where would snow have come from in July?

A movement behind the trees prompted her to investigate the strange occurrence. As she approached, she heard laughter.

"You missed!"

She followed the young voice and found a boy with unnaturally white hair who hid behind a tree and then swiftly dodged out of the way to avoid getting hit by a snowball. The source of the assault was a young girl with long braided hair as white as the boy's. Her hand was empty at first, then a snowball appeared in it which she immediately threw. The boy ducked and propelled a snowball back, hitting her in the forehead.

"Got you! Ten to five. I win!" he yelled and ran up to swing her around in a hug.

They both laughed but she secretly created a snowball behind his back and smashed it right into his face. He let go of her but did not stop laughing.

Minerva brought her hands together, delighted at the sight. It was common for young wizard children to be able to create some wandless magic but it was usually accidental and limited. In all of her years, she had rarely witnessed this level of control over wild magic. These two children were very skilled and she was impressed.

The girl was the first one to notice her. The boy first dismissed her but then did a double take and also stared. Minerva realized that they might have thought that she was a Muggle who had just witnessed a magical snowball fight so she approached them and tried to smile pleasantly.

"Don't worry, I'm not a Muggle." Her explanation did not take the shock out of their expressions. "You're not in trouble, I just wanted to talk to you."

The two kids exchanged looks, and some silent agreement went on between them. The girl appeared cautious but the boy looked back at Minerva with a carefree lopsided smile.

"Hi," he said.

Seeing them up close, she noticed that the resemblance between them stretched to more than just their hair. They were both pale, had similar facial features and identical icy blue eyes. The only difference between them, besides the gender, was that the girl was about an inch taller. Minerva guessed that they were siblings.

"That was a very impressive wandless magic and I'm wondering where you learned it. I'm a Professor at Hogwarts and don't remember ever seeing you there."

The boy frowned at her words but then just shrugged. "It's what we do."

Minerva noticed that he didn't answer her question at all and she supposed that maybe they weren't of school age yet.

The teacher in her worried about them. She appraised them and did not like what she saw. Their Muggle clothes were clean but very worn. The girl wore a faded teal dress which was frayed at the bottom, and mary-jane shoes which were so worn out, she couldn't tell what color they were anymore. The boy wore a blue hoodie which had seen better days, pants which were ripped on both knees and no shoes at all. He saw her staring at his bare feet and rubbed the back of his neck nervously.

"I don't like shoes," he said but Minerva did not buy it.

She felt like giving a piece of her mind to their parents for taking such poor care of them.

A clattering noise sounded behind them and she looked around to see a street vendor who set up a hot dog stand in the park. The boy watched the vendor longingly and his stomach grumbled loudly.

He shifted his feet in embarrassment but then addressed his sister, "Are you hungry?"

The girl dropped her gaze to the ground and he nodded in understanding.

Minerva's heart swelled. She didn't get to be a mother but had witnessed this situation when visiting her brother's family. When a mother felt cold, she'd put a sweater over her child. When a father felt hungry, he'd make food for his children. This boy had the same type of relationship with his sister and it was heartwarming to witness.

"If you don't mind, we need to get going. It was nice talking to you," the boy said.

"Bye," the girl added with a little wave and followed her brother.

Before Minerva could say anything, they left in the direction of the hot dog vendor.

She had a feeling in the pit of her stomach that she couldn't quite explain. Her intuition told her that she shouldn't let them go just yet, so she followed them, keeping her distance. Very soon, she found that her instinct was correct.

The vendor was preparing two orders of hot dogs and put the finished food on the counter so that he could accept money from a customer. The white-haired boy walked up and took the hot dogs off the counter without saying anything or paying, and walked away casually. The vendor rather oddly did not notice the thief himself, but became very confused by the missing hot dogs and began arguing with his customer.

That little thief! Furious at what she had witnessed, Minerva followed the boy to the bench where his sister waited for the stolen hot dog.

The boy startled from seeing her in front of them.

"Hi, again," he said with his mouth full.

The girl gave her a little smile and a wave.

"You… you just stole this food!"

He frowned and then shrugged.

"We were hungry," he explained as if that was supposed to excuse the behavior.

"Then, buy the food. Don't steal it!"

"What do you suggest? That we starve? We do what we have to," he declared defiantly and took a big bite of his hot dog, daring her to contradict him.

At first, Minerva wanted to scold him further, but then the meaning behind his words struck her. She looked at how skinny they were and at their haggard clothes and felt like someone punched her in the heart.

"Where are your parents?" she probed, trying to not sound as harsh.

"Don't have any. Don't need any."

He finished his hot dog and threw the wrapper into a nearby trash can. He wiped his mouth with the back of his hand and stared at the hot dog vendor again. Minerva guessed that he was still hungry.

"Who takes care of you? Where's your family?"

The boy crossed his arms and leaned back. "Elsa is my family. I'm hers. I take care of her and she takes care of me. We're all the family we need. The question is, ma'am, why do you care? Just let us be."

He just confirmed her suspicion. They were on their own. Were these children orphans or runaways?

The girl finished her hot dog, took his hand and they started walking away.

Why did she care? Because it was the right thing to do.

"Wait," she called out after them and walked fast to catch up, her heels making a clacking noise on the pavement. "You shouldn't be alone, children. There are alternatives."

The boy snorted. "Yeah, right."

"I could help you find a good wizard family to take you in."

"Wizard family? As in wizards?" he asked with a raised eyebrow.

He seemed to be amused by this and Minerva didn't quite understand why. They had to have known that they were wizards, right? The idea that they didn't know seemed so strange to her.

"You do realize that you are not Muggles. Do you?"

"What are Muggles?" the girl asked curiously.

"Non-magical people. You have magic. You're a wizard and you're a witch." She had a hard time believing that no one had ever told them that yet.

To her surprise, the boy snorted and tried to suppress giggles. The girl exchanged a look with him and giggled as well.

Minerva didn't know what to think of it. They didn't know? How could this have happened? They looked to be somewhere between ten to thirteen-years-old already. It was obvious by the ease with which they used magic that they had these skills for a while. No one noticed? Improper Use of Magic Office was usually very good at detecting underage wizardry. How did they miss these two?

She needed to get some more information out of them if she had any chance of helping so she decided to give them an offer they couldn't refuse.

"I bet you're still hungry."

Two white heads snapped to attention and she tried to not look too smug.

"I'll buy you both proper meals if you agree to talk to me about your living situation."

They looked at each other and a silent agreement was made because the boy smiled at her and said, "Just talk? Deal."

Author's note: How do you like the story so far? Let me know - your feedback is what keeps me going. You don't need to have an account to leave a review, but if you do, I can respond - I reply to all.

Coming up next, we meet Merlin!