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If Arthur knew beforehand how much mental and emotional stress he would be getting as a result of being Merlin's friend, he would have told his younger self to steer well clear of the skinny first-year with dark hair and blue eyes no matter what.
He didn't know that there were this many unicorns in the world, let alone in the Forbidden Forest. He watched, dumbstruck, as yet another pearly white creature trotted eagerly out of the shadows, joining the already massive crowd of their fellow brethren that milled around Merlin, making happy puffing noises and gently headbutting the Hufflepuff. Where were they all coming from?
"I thought they didn't like boys," he said, after finding his voice again. Merlin snorted, his hands busy trying to pet twenty unicorns at once.
"They're really not that bad. Do you want to come and say hi to them? I'm sure they'll be happy to see you, too."
But I've never met them? Arthur edges slightly closer towards the castle grounds, shaking his head and eyeing the two foot-long spiralled horn on the largest unicorn's head. He may be a Gryffindor, but unlike Gwaine, he knew how to pick his battles.
"I'm good, thanks."
Merlin shrugged (as much as he could shrug, almost smothered under the weight of twenty affectionate unicorns). "Suit yourself."
"Sometimes I wonder if you're human," Arthur said half-jokingly, quickly backing up as a twenty-first unicorn galloped out of the woods and joined the fray.
It wasn't just with unicorns, either. Arthur had only run into the centaurs once - a detention with the caretaker, patrolling the outer rims of the Forbidden Forest - but they seemed to know him, which was very suspicious, not to mention the fact that at the sight of Merlin, who was stumbling through the woods trying to catch up after having tripped on a tree root, they all simultaneously bowed.
And Arthur might not have met a centaur in his life before that, but he does read, and he knows that centaurs are extremely proud, territorial, and have a rather deep contempt towards humans. And they definitely don't bow to random kids.
"Do you know these guys?" the Arthur asked uncertainly. Awkwardly, Merlin looked around.
"Um," he said unconvincingly. "No?"
In fact, it wasn't just the unicorns and the centaurs (otherwise Arthur would have jumped to the conclusion that all living things with four hooves just happened to mysteriously love Merlin at first sight) but the entire Forest. Merlin had followed Arthur when he went to try and rescue Gwaine and Percival from the clutches of an Acromantula-infested nest (to this day, he still doesn't know why there were in there in the first place) and the massive spiders towering above them scuttled away to make way for them. He'd heard multiple warnings not to go into the forest alone, that rogue Dementors still lingered there and would attack any defenceless person they saw, but he'd walked through those woods far too many times, and while he was with Merlin, the most he'd ever see was a wisp of shadow slinking away behind a tree.
When he was with Merlin, the entire Forest seemed alive, and it seemed to worship them. Like they were kings.
"This is very suspicious," he told Merlin once. Merlin looked over his shoulder, towards where they had just left the Forbidden Forest. The trees looked as dark and gloomy as ever, but there was a certain light about the place, and the wind in the creaking boughs almost made it look like the branches were waving them goodbye.
"Do you not like it?" Merlin asked in reply, in that odd, enigmatic way he sometimes had. Arthur pondered for a while, then shrugged.
"It's not bad, I guess."
Then, of course, there was that one time that Arthur followed Merlin out into the woods again (because he was curious about why Merlin was sneaking out at four in the morning, and was looking for an excuse to give him a detention with his new Prefect status - not because he was worried about his safety, or anything) and stumbled across the unexpected and very alarming sight of Merlin standing in the middle of two massive dragons.
"Oh, fuck," he said aloud, before realising that wasn't a good idea. Both dragons slowly turned their heads towards him.
The smaller dragon of the two was pure white, with blue eyes.
Arthur squinted.
"Wait -"
Aithusa made a quiet rumbling sound in her throat.
"Greetings, young king," she said serenely. Arthur gaped at her, then spun around to face the larger dragon. It was a huge creature, with a much more ancient look to it, dull gold scales and yellow eyes.
"Wait - aren't you that dragon from the school trip? Why are you here?" Arthur paled, his hand automatically going towards his wand. "Oh, god, Merlin, did you let the all dragons out?"
Merlin looked just as bewildered.
"What? No! Kilgharrah just decided to come by - why are you here, anyway? What are you doing sneaking around in the middle of the woods?"
Arthur relaxed a little, now that it was established that neither of them were at risk of being eaten, and crossed his arms, looking at Merlin accusingly.
"I was following you, what were you doing sneaking around in the middle of the woods?"
The large golden dragon stirred, looking at the scene with a distinctively un-dragon-like appearance.
"I seem to remember this scene," it said in amusement. Merlin scowled at it.
"Kilgharrah, shut up."
Arthur started.
"You can talk to that dragon too?"
Looking way too calm for a person surrounded by talking dragons, Merlin shrugged. "Technically, I can talk to all dragons -"
"What?"
"- though most 'dragons' nowadays are lesser dragons, so they can't actually speak. These two are Greater Dragons. If you know how to talk, you can talk with them too."
Arthur looked back and forth between the two dragons, who seemed to be taking equal delight in his confusion, and Merlin, whose expression was flickering between apprehensiveness and entertainment, and sat down heavily on the grass. Frowning, he pointed a finger at Merlin.
"You have a lot of explaining to do."
"Oh, would you look at the time, it's almost breakfast - we should get going, back up to the castle - Aithusa, let's go -"
"Merlin!"
Arthur, despite all the suspicious activities and actions he'd seen, would still have probably been willing to forget all of it for Merlin. He could have accepted the theory that Merlin was not, in fact, fully human, but maybe the child of some nature spirit of some kind, a half-dryad or something - after all, that would explain his odd magic, his affinity with magical creatures, and a list of other off things about Merlin that Arthur had carefully filed off in a list in his mind. But the best thing about that theory was that it had everything to do with Merlin and nothing to do with Arthur. Arthur could just remain a normal human Hogwarts student, with a half-nature-spirit-slash-dryad best friend.
Of course, life never made it easy for him, and only a few days after the dragon incident, Arthur accidentally made the discovery that he could apparently speak a dead language.
It was all Aithusa's fault. Arthur was sitting in the Hufflepuff dormitories, waiting for Merlin to return from the library, and Aithusa was sitting on his bed, licking her paws and giving him a look that felt almost mocking.
You seem troubled, young king, she said in his mind. In his mind!
"How the hell are you doing that?" Arthur replied grumpily, having half a mind to yank the blanket just to have the satisfaction of seeing the ever-calm cat (dragon?) lose its balance. "And stop calling me young king. Just because I have the same name as a legendary Muggle figure -"
Aithusa padded closer, settling down inches away from his lap.
I am one of the last Great Dragons in England, she said, somewhat proudly. Even if you weren't in possession of the magic of old, I still would have been able to talk to you like this. And I call you by your title. Is that not normal?"
How did nothing this dragon ever say make sense? "What do you mean, magic of old? Is there a new kind of magic going around? And just because I'm a pureblood, it doesn't mean I'm literal royalty - I'm not some magic supremacist -"
The bed curtains were yanked open, and Arthur yelped. There was someone standing there - they were in pyjamas, but with a Hufflepuff tie hanging loosely around their neck, and Arthur distantly recognised him as one of Merlin's dormmates - and Arthur's heart immediately started hammering. How much did he hear?
But then -
"Jeez," the Hufflepuff said in amazement. "What language was that?"
"... What?"
"That language you just spoke. That wasn't English - was it Welsh? It kind of sounded like what Professor told us old runes - was it Elder Futhark, or something? What they sounded like -"
Edging away slightly, Arthur cast a suspicious glance at the white cat on the bed.
"I'm pretty sure that was English?"
"No, that definitely wasn't."
Aithusa miaows, and headbutts Arthur. Arthur stares at her incredulously.
"Were you talking to me in Welsh?" he whispered.
Not quite, he got in reply. It's a dead language, a form of Old English, spoken about fifteen centuries ago.
"... And how the hell do I understand Old English?"
Aithusa just sat down and began licking her paws again, completely ignoring the rest of his questions. Arthur should probably have questioned this odd quality of his further, but at this point, after having dealt with Merlin and his weirdness all the way through their Hogwarts years, he just filed it away under the ever-growing list of "What the fuck" and didn't think about it for the rest of the day.
Arthur paced up and down the library, running a hand through his hair and ignoring the disdainful looks from the librarian.
"When did I learn an entire language - a dead language, mind you - without noticing? How the hell did that happen?"
Merlin was absently patting Aithusa with one hand while doodling in the corner of his astronomy assignment. He looked up, noticing Arthur's mild distress, and shrugged, looking rather unbothered by how concerning his situation was.
"If it helps, I read an article once about a muggle getting a concussion and waking up speaking fluent Spanish?"
"That doesn't help, Merlin!"
"It can't be dangerous," Merlin said consolingly. "Look - now you'll have an easier time with History of Magic, won't you? Maybe your father just read you Old English books as a baby to try and make you bilingual. That does sound like the kind of stupid thing Uther would do."
"... I mean, you're not wrong, but you don't have to say it so insultingly." Arthur shook his head, sitting down heavily in the chair next to Merlin. "Anyway, it's not just me, is it? Aithusa also knows how to speak Old English! How does she know? And - wait, you speak to her the same way - do you know Old English?"
Merlin gave him a funny expression. "Aithusa's a dragon, Arthur. She's probably old enough to have lived through the medieval period, so she probably knows Old English from back then."
"And you?"
At that, Merlin glanced away shiftily.
"That, er - Aithusa taught me."
Arthur did not trust that answer one bit, but Merlin was looking uncomfortably close to running away, so he decided not to bring up the suspicious answer again.
"You know where the Mirror of Erised is?"
Merlin frantically waved his hands, shushing him and glancing around panicked at the rest of the rather empty library.
"Shh! Be quiet! Nobody's supposed to know about that!"
Arthur gaped at him. Really, he was learning something new every day when he was with Merlin.
"But - you - how? It's in this school, right? Show me where it is, I've always wanted to look into it -"
At that, Merlin's expression changed drastically for a split second - a weird mix of fear, hope, and crushing doubt - before it settled into a familiarly exasperated look.
"Have you read anything about it? It's dangerous, Arthur, half the people who look at it go crazy -"
"Because they were Muggles and didn't know what it did! We'll be fine - come on, show me where it is. I'll do your Astronomy homework for you."
Merlin stared at him open-mouthed, disbelieving. Arthur stared back eagerly, and after a few moments, Merlin sighed.
"Just once," he said finally. Arthur triumphantly slammed shut the homework he hadn't been doing for the past hour, and sped out of the library.
Merlin led him down a series of twisting corridors, upside down staircases, and hallways that seemed to stretch on for far longer than they possibly could, and just when Arthur was beginning to wonder whether he was just making this all up, Merlin pushed open a door that he was certain wasn't there before, and led him inside.
The room was empty, stone walls covered in a thin layer of dust and cobwebs, and although he was pretty sure they were underground, there was a small barred window where a single ray of weak sunlight pooled onto the floor. In the centre of the room, there was a large mirror.
"... is that it?"
Merlin hung back, lingering near the doorway. He gestured hesitantly towards the mirror.
"Yes. Don't look at it for too long, Arthur. I mean it."
Maybe it was the eerie atmosphere of the room that looked as if it had been abandoned for centuries, or it might have been the fact that the route that Merlin took was so winding that Arthur didn't actually know where they were in the castle anymore. Either way, the excitement that he had initially gone looking for the mirror with was suddenly dimmer, and he approached with a feeling of trepidation.
For the first few seconds, he only saw himself. School robes, Gryffindor Prefect badge, that one small burn on his left sleeve from this morning where he wasn't paying attention in Potions and singed his clothes. (It was Merlin's fault.)
And then -
"Oh my god -"
He thought he would only see one image, but there were several, flicking rapidly between each other like a slideshow that was too fast for the normal eye to see, yet he could still make out every detail. There he was, an older version of himself - an Auror? Uther's face in the background, for once proud and not disappointed, and his mother, a figure that he only recognised from old photos and paintings. There was everything - success, family, fame, happiness.
But the longer he looked, the more bizarre the images became.
First there were forests. Vast landscapes of green uninterrupted by any sprawling cities of concrete, stretching blue skies and gaping caves. He didn't think much of those - he's always liked playing in the little forest outside his family's estate, but these forests seemed different, and there was so much of it. Then there were castles, great structures of grey stone and fortified walls, unfamiliar flags flying from masts that still seemed like home. Bustling villages, armies, magic in an unfamiliar form. A small group of men in armour and red cloaks. A young man with a worn brown jacket and a scrap of red fabric at his neck.
Merlin, his mind supplies helpfully.
But that was ...
Why was the mirror that was meant to reflect his deepest desire showing him scenes from a kingdom that looked as if it had existed hundreds of years ago?
... nothing made sense in this castle.
"Arthur?" Merlin's voice floated towards him from the doorway, seeming weirdly distant in his ears. "You haven't gone crazy, right?"
Shaking his head, Arthur looked back at the mirror. The kingdoms and armies of old were gone, and there was just him.
"I'm fine, Merlin," he replied. "Honestly. Hey, why don't you have a look?"
Merlin shuffled into sight, looking suspiciously shifty. Arthur knew this look - it was the look he wore when he accidentally let slip that Aithusa was a dragon, or that he could speak Old English, or when he pretended that he hadn't just commanded four Dementors to go away without a wand or spell. It was the Merlin-is-hiding-something look, and Arthur, once again, wondered just how much he really knew his friend.
"I don't really feel like it," Merlin said.
"Come on. Don't be such a girl about it. I'll tell you what I saw."
Merlin cracked a smile. "Oh, I can guess what you saw."
Arthur figured that he probably could (Merlin was there during his many hours of debating over whether he should try and become an Auror like he'd wanted as a child, or whether he should stick with the safe route and take the Ministry job that his father wanted him to take) but he was also getting that weird feeling that Merlin was doing that thing where he said one thing and meant two different things at once. He mused over it for a moment.
"I bet you couldn't. Have you already looked in the mirror? You know where it is, you've probably already seen it. Haven't you?"
Smiling wryly, Merlin shook his head.
"I didn't see anything."
"What do you mean, you didn't see anything? The mirror works for everyone."
Merlin lingered in the doorway for a few more seconds, then sighed, and approached. Arthur glanced towards the mirror in mild anticipation - he knew that he probably wouldn't see what Merlin saw, but he looked anyway. Merlin looked into the mirror, and -
A sharp noise split the air, and the surface of the mirror cracked in half.
Arthur flinched violently, and his wand was already halfway out of his pocket before he realised that there was no danger. Bewildered, he stepped towards the mirror, reaching out and tracing his hands over the crack. It split the mirror right down the middle, a jagged line that tore the happy illusion into two.
He was speechless.
"... how ...?"
How did that break? Wasn't the mirror a powerful magical artefact? They hadn't done anything to attack it, and they were the only two people in the room. Then ...
What sort of desire did Merlin have that would make such an ancient magical mirror break like that? Was the yearning so terrible or deep that the mirror couldn't possibly portray it?
Merlin stared at him silently, and his eyes seem to search his, looking for something that apparently he didn't find, because he let out a little huff and turned his gaze away.
"Yeah, that's why I don't look at it."
He started to reach out his hand towards the mirror, abruptly changed his mind halfway, and pulled out his wand instead. The mirror mended itself with a short flash of light. If his mind wasn't still reeling, Arthur would have commented on the suspicious ease with which Merlin had just performed that wordless magic. (Not only was he powerful enough to break a powerful magical artefact, but he could also mend it at the blink of an eye?)
"It's fine," Merlin said mildly, as if that entire incident hadn't been a big deal. "I don't need the mirror. Most of us don't, you know? We know what we want. It's why we wake up every morning."
And there it was, one of those random moments where Merlin unnerved him by displaying a sort of wisdom that Arthur had only ever seen from the portraits of the old headmasters and grand wizards up in the Headmaster's Office.
"Yeah, I guess." Arthur fumbled for a few moments, unsure of what to say in this odd atmosphere that had descended upon them. "By any chance, your deepest desire ... is it to break every mirror in existence? Do you have something against mirrors?"
Merlin snorted, the corner of his lip twitching up into a familiar grin, and the air was suddenly lighter again.
Arthur followed Merlin out of the twisting maze of staircases and corridors, and although he tried his best to remember the way, he had a feeling that he wouldn't be able to find that mirror again without Merlin leading him.
But then, he figured, staring absently at Merlin, who was walking slightly ahead of him and chatting about some mundane prank that happened in the Hufflepuff common room the other day, maybe he would be fine with that, too.
That night, he dreamt of towering castles and red banners and golden dragons, and the feeling of swinging a sword and the weight of armour around his body lulled him into the best night's sleep he'd had in years.