It's still a bit early for Christmas proper, but I've come up with this idea and just thought, why not. So here's a story set in the first year of HP & co. at Hogwarts, right around this time of the year...
A Hogwarts Christmas
Hermione Granger's time at Hogwarts had been some of the months most full of wonder and discovery of her entire life - safe for the first six of her life, during which pretty much everything has been wonder and discovery, but that she sadly did not remember. She had seen magical creatures, ghosts, and talking paintings; she had learned to cast magic and her best friend flew on brooms for a sport activity; she had friends, by the way (that, in itself, was a pretty novel experience); she had fought a mountain troll, and one of her professors was literally able to turn into a cat. So it was no understatement to say that her world view had been upended, and that her mind was now a great deal more open to new ideas than it had been only last Christmas, before she got that fateful letter.
That said, all of her new open mindedness did not mean she was ready to hear what she was about to hear.
It all started while she was in the library (not an uncommon occurrence) studying some particularly complex and arcane transmutation whose purpose was to turn radishes into beetroots. It did not seem very useful at a glance, but one could never know. But then she had heard a great deal of noise, raised her eyes, and saw two identical redhaired heads bolting through desks and chairs towards her. Her friend Ron's twin brothers: Fred and George Weasley.
They were aiming for her. There was no reasonable escape route. She had to face them head on.
"Hello." she greeted them. "It's a nice day, isn't it?"
The twins exchanged a disheartened look, shook their head, and sighed. Fred (or was it George? She decided she'd go with Fred. He looked like a Fred) leaned in, wrapped an arm around her shoulders, and whispered in her ear in a very conspiratorial fashion. "Hermione Granger, we need your help."
"My help?" she said, surprised (and perhaps a bit flattered). "What for?"
"Well, everyone says you're the brightest witch of the first year." said Fred.
"And a couple of years above that too." added George.
"They say that?" asked Hermione, now definitely flattered. And blushing for it.
"Well, they don't say it." specified the boy. "But they think it."
"Anyway, we're here to seek your help, o wise one." jumped in Fred. "For, you see, we need you to come up with-"
"A devious device." suggested George.
"A practical prank."
"A treacherous trap."
"A cunning cuntery."
"Excuse me!" Hermione jumped up to her feet, outraged. George slapped Fred on the back of his neck.
"Brother, that's no language to use with a first year!"
"Sorry. Got caught with the flow."
"Anyway," continued George, "we need you to help us. Over the Christmas holidays. Only you can do it, o youngest of the wisest!"
"I'm... pleased that you hold me in such high esteem." said the girl, politely. "But I will go back to my home for Christmas. I can't stay in Hogwarts. Can't I help you at another time?"
"Oh, but no, you see, it has to be Christmas." said Fred, whispering. "Because, it's about, y'know..."
They looked around, then at each other, then at Hermione. Then they pointed a finger above.
"The ceiling?" asked Hermione, confused.
"No, not the ceiling. It's about him." said Fred. "I can't mention him out loud."
"It's dangerous." confirmed George.
Hermione took a hand to her mouth. "You mean... You-Know-Who? I thought he was dead."
"Best of our knowledge, he is." muttered Fred. "But this one..." and he shook his head.
"He sees you when you're sleepin'." explained George.
"He knows when you're awake." added his brother.
The girl sneered, with the most patronising sneer she could muster. "You're not seriously telling me you're talking about Santa Claus, are you?"
The two were caught by a fit of panic and scrambled to physically cover her mouth.
"Don't-say-that-name!" said George, alarmed.
"Call him The Red Fatso." intervened Fred, helpfully. "Much safer."
Still more surprised than angry, Hermione tapped their hands to signal that she had gotten the drift, and after a second of reflection and a quick agreement, they removed them and let her speak again.
"Ok, you're not seriously telling me you're talking about-" Hermione had to stop for a moment to snicker "-The Red Fatso, are you?"
"We seriously are."
"This is as serious as it can be."
"Not that we're not serious most of the time anyway."
"Yeah, right." she glanced at both of them with extreme annoyance. "I'm eleven, you know. San... The Red Fatso is a myth."
"A myth, sure." said George. "Like wizards and witches and unicorns and dragons."
The girl huffed. "Just because I'm Muggle-born doesn't mean you can just try to make me believe anything. No book mentions anything about him."
"Not everything is in books." suggested Fred.
"In my experience, if something is not in books, it's not very important."
"That's Muggle thinking." commented George, shaking his head. "For wizards, if something is not in books, it's usually most important."
Hermione sighed and realised this would not end until she humoured the two brothers. So she put up her best impression of a teacher just ready to drop a bad grade on you if you only as much as say a single mistaken word, crossed her arms, and sat back. "Fine." she said. "Convince me."
"The Red Fatso is a dark wizard." said George, gravely. "In fact, one of the darkest."
"Thing is, we've discovered his identity." explained Fred. "And now we want to render a service to the world, by ridding it of his presence."
"Let me get this straight," asked Hermione, "you're asking me for help in killing Santa Claus?"
"THE-RED-FATSO!" shrieked the two, so suddenly that she jumped back for the sudden scare. They seemed indeed very worried, she had to admit. They were taking this awfully seriously for a joke, even for their standards.
"Sorry." she found herself muttering.
"Let's hope you won't be." George looked at her sadly. "Now, maybe you don't believe us because you don't know about the Donatus Charm..."
"The Donatus Charm? Never heard of."
"The Donatus Charm," explained Fred, "is a bit like the Fidelius. It's one of those charms that require certain conditions and tricks to activate."
This made sense to Hermione. Maybe there was something to learn from all this ridiculousness. "And what does the Donatus Charm do?"
"It binds the will of the person it is cast upon. But unlike the Imperius, it is not forbidden because of the complex conditions of activation." continued the boy. "For the Donatus Charm to work, the victim needs to accept a charmed gift from you, willingly, for seven years in a row, every time on the same day. Upon which, the contract is complete, and the victim magically owes a favour to the caster, who can exact it at any moment, in the form of a single order that will be obeyed without any question."
This, to Hermione, sounded disturbingly like the kind of warped logic that magic would follow. "Seven years, you say?"
"Ages three to ten, usually." explained George. "Kids all over the world. Not all of them last that long, but a great deal do. Enough for a huge, loyal Muggle army once they're grown up."
"That doesn't make sense." snapped Hermione. "Not even a wizard could deliver all those gifts in a single night."
"What, you really think he uses a flying sleigh?" Fred rolled his eyes. "And then what, Rudolph, the poor, bullied red-nosed reindeer pulling it?"
The girl pouted. "Well, you're the ones trying to convince me that The Red Fatso exists. And must be killed."
"I thought you were the clever one." said George, with a tinge of disappointment. "What travels instantly anywhere in the world and makes things appear in a fireplace?"
"...Floo powder." whispered Hermione, suddenly hit by the realisation. "But I thought the Network didn't reach Muggle houses."
"There used to be older Networks. Forgotten ones." continued the boy in an ominous voice.
"Ok, I'll give you that it's a possibility. But what about the toys? Am I to believe that The Red Fatso is a dark wizard, spending his whole year at the North Pole, having all the work of making billions of toys left to his army of faithful..."
And Hermione's voice cracked.
"...elves." she finished, in a whisper.
"She really is quick on the uptake after all." commented Fred.
She could not believe it. But it was starting to make so much sense.
"But the parents!" she cried desperately. "They lie knowing they're lying! They remember buying the presents!"
"Don't be naive, Hermione." George shook his head. "Confundus. Obliviation. False memory charms. So many ways."
"So many lives forever warped." added Fred.
"Minds tampered with. Perhaps damaged."
"So much that could have been lost forever."
"The thought is unbearable."
Hermione was gripped by a sense of terrible dread.
You better watch out, was the ominous, sinister hiss she thought she could hear, bouncing in her head. You better not cry.
But she felt like crying indeed. Fear and pain were swelling in her chest.
"Who is it?" she whispered.
"You already know. You're so well read." said George.
Yes, she already knew. Saint Nicolas, called him the Muggles, which had become Santa Claus under the weight of centuries of language mutations. An immortal wizard named Nicolas.
"Flamel." she said. "The alchemist."
The twins nodded.
"I thought he was a good guy."
"Everyone does." explained Fred. "That's what makes him so dangerous."
"Oi, you idiots!" screamed someone, from a distance. "What the heck are you doing to Hermione?"
And suddenly a third redhead crashed the scene. Ron, one good foot shorter than his brothers but more than making it up by amount of noise he could produce when he decided he was pissed at them.
"Blimey, what have you been telling her?" he asked, after a glance at Hermione. "She's shocked!"
"Just the bitter truth." said George.
"The one you all are in denial about." added Fred.
"They told me about Sa-" started explaining Hermione, and then caught her own mouth. "The Red Fatso." she concluded.
Ron looked at her with an incredulous stare, then he started laughing loudly. He bent over himself laughing, all while Hermione gradually shifted from scared, to confused, to outraged, to ashamed, to even more outraged, to just wanting to go hide somewhere and never surface again and die.
"Y-YOU!" she screamed, barely understandable, at Fred and George. Then they didn't explain themselves, or did, she didn't know. She was crying at that point.
"Oi, Hermione, don't take it that badly. They do it all the time. First years Muggle-borns, it's a yearly tradition by now. Should have warned you."
He threw a judgemental glance while he was pulling Hermione, still crying like a fountain, away from them.
Fred and George looked on, dumbfounded, while the two first years left the room.
"Ah, youth." commented Dumbledore, who had observed the scene from the gallery going all round the first floor of the library. "Such mirth."
"She didn't look very mirthful." observed the old man next to him, whose dress managed to somehow be even more eccentric than the principal's.
"She'll recover. Quick to cry, quick to laugh, such is their age. Lemon drop?"
"You just carry those loose in your pockets?" asked the other.
Dumbledore didn't flinch. "Why, yes."
"Thanks, I'll pass."
They walked a bit more, admiring the immense book collection that Hogwarts had to offer. Too bad most of its occupants didn't care much for reading or studying. The last time a student had bothered actually trying to read a significant fraction of it he'd almost conquered Britain single-handed.
"So," asked the old man, "is it safe?"
"As safe as it can be." answered Dumbledore, smiling. "Warded by the most sophisticated and deadly traps my professors could come up with. I've also informed all of the students of the exact location of said traps. Here at Hogwarts, we take safety seriously."
"Uhm." the other massaged his beard. "That's comforting, I guess."
"So, Nicolas!" exclaimed the principal, with a jolly expression. "Care to join us for Christmas dinner since you're around? Our kitchen serves a delicious pudding."
The old alchemist looked genuinely sorry. Only the most attentive observer, the most discerning judge of character, could have caught the faintest glint in a corner of his eye.
"It's a pity," he said, "but on Christmas I already have something to do."