Hermione, Neville and Dean arrived back by train with the rest of the students on the third of January, a few days before the spring term began. The castle was just as deep in snow as before, and Hedwig was quite smug about how easy the snowstorms were for her relative to the other owls.

Most of that evening was taken up talking about presents – Neville's spider plant went on his bedside table, while Hermione had already reached the sixth century. Harry proudly demonstrated his Weasley Jumper, Ron recharged the griffin he'd got in the cracker and had it do a few tricks, and Dean somewhat reluctantly showed that his mum had got him a dustpan and brush.

"She said it was because first years weren't allowed broomsticks," he said.

"Is this what Muggles use instead?" Neville asked. "Do you put your feet on this bit and… or not?"

"No, it's a joke about cleaning," Dean explained. "I got some books as well, this series called Redwall."

"I've read one of those," Harry told him. "It was pretty good. Scabbers might not like them, though, rats are the bad guys."

He frowned, sitting back a bit. "But I know orcs are the bad guys in The Lord Of The Rings, maybe some real orcs aren't?"

"What's an orc?" Ron asked. "I don't think there's such a thing as an orc."

Harry shrugged, a big expansive gesture with his wings. "We don't have a textbook called Fantastic Beings."

"That's because it's a lot harder to describe Beings," Hermione said, looking up from Belisarius' North Africa campaign. "A lot of Beasts are the same as one another, but Beings are really varied and not really a good subject for a first-year textbook. I think they turn up in History of Magic and in Muggle Studies."

"Why would they be in Muggle Studies?" Neville asked.

"Because Muggles know about a lot of them," Hermione answered. "Or they have myths at least. Like… giants, they know about giants, and goblins as well. And in Japan there's several stories about kitsune. But I don't think Muggles know about veela."

The conversation went on from there, and Hermione put a bookmark in her book to properly join in the discussion. They talked about whether Harry counted as a Being or a Beast, and whether Wolfsbane would mean that a Werewolf counted as a Being while transformed if it had taken the potion, and how it wasn't really fair that a lot of Beings weren't allowed wands.

An odd mention in one of the books they got out to check on their facts also left Harry wishing he could head over to Ravenclaw Library, but it was after curfew so he'd have to wait – and that reminded him of the other thing he wanted to tell his friends about, the Erised mirror.

It mostly wasn't that he wanted to go and see it again. It was more that… well, after Ron had looked into it, he'd been a bit thoughtful about what he'd seen.

Even if just staring into the mirror was a bad idea, and even if his desire wasn't something he could ever actually have… Ron's desire (which they'd talked about and decided was to go somewhere no wizard had ever been) was something he could actually manage, if he put in a lot of work on it.

Would the same thing happen with his other friends? It might, and that was what Harry thought was worth knowing.

Besides, it was really neat.

The next day was a Saturday, and Harry led his friends (all four of them, with Ron coming along for what he called 'moral support') through the castle to the disused classroom.

"Are you sure this is allowed?" Neville asked.

"Percy said that anywhere we're not supposed to be is either password locked or just plain locked," Harry replied. "And… here."

He pushed the door open, then frowned.

Ron pushed past, and saw what Harry had already noticed – the mirror was missing.

"That's kind of a shame," Hermione said. "I was actually interested in trying to work out how this magical mirror works."

Harry frowned, and sniffed the air a bit.

"I think someone moved it," he supplied. "Maybe yesterday? We could see if we can find it."

"But if someone's moved it, could it be because we're not supposed to look at it?" Dean asked. "Only makes sense to me, if there was something kids weren't supposed to look at."

"That's easy," Harry replied. "Like I said, anywhere we're not allowed to go is going to be locked, so if we run into a locked door then we're not allowed through. Simple."

Hermione nodded, agreeing that that made sense, and they followed Harry back out of the classroom as he did his best to track down the way the Erised mirror had gone.

They went along a corridor, then down a flight of steps, and after that through a secret passage that started behind a tapestry, sloped down, and came out again through a sliding wall.

"I told my mum about how many secret passages there are here," Dean said. "She didn't believe me."

"It's kind of common in old magical buildings," Neville told them. "My bedroom's on the third floor, but there's a way to get to it where you go to the first floor landing and then down a flight of steps."

A couple of fifth-years went past, telling one another something like 'it's still there' and giggling, and after another corner they came to a door.

Harry sniffed a bit, and nodded. "It definitely went in there," he judged, and pulled the door open.

The first thing he saw was three giant canine heads, bobbing up and down as they looked at him. One of them growled, the third one roared, and then the one in the middle shook itself and bumped both of the other heads.

"Oh, goodness me!" the middle head said, as Harry saw that all three heads were attached to the same giant dog. "You must be that young dragon that Rubeus talks about."

"So it is!" the left head realized. "Delighted to meet you, dear boy."

The three-headed dog offered a paw to shake, and Harry did so.

"What's going on in there?" Hermione asked, and Harry used a wing to push the door open.

"Oh, are these your friends?" the middle head asked. "Come in, come in, it's lovely to meet you all. You must be that Hermione Granger girl, Rubeus is very impressed with you – oh, and that's Ronald Weasley, isn't it?"

All four of Harry's friends came in, and Dean shut the door behind them as the big dog continued talking.

"So, tell me, how has your term gone?" the left head asked. "I'm afraid we haven't had a chance for a magical education ourselves."

"...you're a three headed dog," Ron said, staring.

"Well spotted," the right head grumbled.

"Oh, calm down," the middle head told him. "But he does have a point. I'd have thought that friends of a dragon would be quite happy with talking to a three-headed dog."

"It's not that we don't like the idea of talking to you," Hermione tried to explain. "It's just… we didn't know three-headed dogs existed. Or do you prefer Cerberus?"

"Either is fine, my dear girl," the left head assured her. "Either is fine. And I'm afraid we're rather rare – you won't find us in any textbook! Rubeus was delighted to meet me, I have to say."

"Why aren't you in the textbook?" Neville asked, beating Hermione to the same question.

"Why, I'm a Being, of course!" the dog explained. "Sadly I only get one vote between the three of us, but we usually agree on what to do with it."

"That's what we were just talking about yesterday," Dean realized, slapping his forehead. "Now I feel like an idiot..."

"Oh, but where are my manners," the left head said. "You can call me Fluffy."

"...really?" Ron asked, still sounding like the whole situation had rendered him completely discombobulated. "Fluffy?"

"That is my name," the middle head agreed, somewhat miffed. "Is something wrong with it?"

"It's a bit of a surprise," Harry told him. "I'm surprised you don't have three names."

"Well, Rubeus is the one who gave it to me, and I do quite like it," the left head said. "Really, though, that is a good point. Perhaps I should think of Fluffy as my surname…"

After half an hour or so of conversation about magical education and what Muggles thought of three-headed dogs, Harry asked about the trapdoor.

"Oh, well, do you know I'm actually supposed to be guarding it!" Fluffy's left head answered. "Why do you ask?"

"Well, we're looking for a mirror," Harry told him. "It's somewhere around here, the scent goes through that trapdoor."

"Well, now," the middle head mused. "A mirror? I remember something like that..."

"Not supposed to go down there," the right head grumbled.

"Full sentences, please," chided the middle head. "Those erudition lessons were for all of us."

"Is it locked?" Harry asked. "We got told that the only doors we weren't allowed past were either locked or password-locked."

"I see, I see," the left head rumbled, sniffing, as the whole of Fluffy stepped back to expose the door. "Well, I don't believe it's locked, now that I come to think of it…"

Harry pulled, lifting the ring of the trapdoor, and it opened with a light squeak from the hinges.

"There's no stairs," Ron pointed out.

Harry shifted his wings, half-opening them. "I'll go down and have a look. Maybe there's an enchanted trampoline or something, and if there's not I'll be fine."

"Or you could just use a light," Hermione pointed out. "Lumos."

Her wandlight revealed a mass of plants, and they exchanged glances.

"I'll go down and check," Harry decided, and slipped down into the hole. Deliberately not using his wings to brake his fall all the way, he dropped down hard and landed on the springy plant matter.

"It's fine!" he called up, then felt the plant still moving after the bounce from his landing should have stopped. A tendril curled up towards his neck, and he bit it off – it tasted sort of like celery. "But – um – hold on, this stuff's moving!"

Neville gasped. "Wait, that's Devil's – look out, Harry! It likes cold and damp conditions!"

"Hyacinthum flammare!" Dean called, sticking his wand through the trapdoor.

When Dean's massive dose of bluebell flames was over, the plant (which Neville clarified was called Devil's Snare) was trying to hide in the corners of the room. Harry himself was half-covered with bluish fire, but it helped him see so that was all right.

The next room contained flying keys, which was very strange indeed, but it also contained brooms, and Harry brought two of them back for the others to join him down at the bottom of the trapdoor in relays – with Fluffy asking them to please be careful, as he wasn't sure what kind of dangers there were down there if there was a Devil's Snare infestation in that room.

"Doesn't this door qualify as locked?" Hermione asked, trying the handle to the next one along.

"The key's up there, though, isn't it?" Harry shrugged, scraping off the sticky magical fire now he didn't need it. "There's loads of them, it's got to be one of the keys… any ideas?"

Ron looked at the lock, describing it, and Harry looked back and forth – then pointed. "There!"

With that sorted out (albeit with a bit of trouble getting the target key surrounded) they next found a massive chess board.

Dean pointed at the chess board. "Are you sure we're meant to be down here?"

"Why would a giant chess set mean we weren't supposed to be down here?" Ron shrugged. "It's like a puzzle. Haven't you seen that door to the kitchens where you have to tickle the right bit of fruit in a painting?"

He frowned. "Okay, so if this is Wizarding Chess, then… can we begin?"

The white pieces on the other side of the room stood there impassively.

"Oh, okay then, we don't need to," Harry decided. "Unless I need to be the one playing on the other side?"

Dean shrugged. "Sounds good to me."

Harry walked forwards, but when he got past the middle the pawns all drew their swords. The stone blades raised in unison, pointing at Harry, and Ron snapped his fingers.

"A-ha! So you can move!"

The white king and queen shifted slightly. They had no faces, but it seemed like they were looking at Ron.

"Don't you know the rules?" he asked. "White goes first!"

The white queen put her hands on her hips, then gestured at Harry, and the dragon loped sideways to get off the board. Once that was done, one of the white pawns moved two spaces, and Ron pointed. "King's knight to H3..."

Harry knew that Ron really liked chess, but by the time they were half an hour into the game – with Ron trying to get a bishop into position to checkmate the white king – he was sort of wishing he'd brought a book.

"Wonder what we'll be doing on Monday," Neville was saying. "I'm not sure I'll be very good, I haven't had a chance to practice over the holidays."

"Why not?" Hermione asked. "It seems very short-sighted if they don't allow you to practice. My family had relatives over the whole time, but if they didn't I'd have shown my parents magic just so they could see what I can do as well as to keep in practice."

Neville frowned. "But... I thought we weren't allowed to do magic over the holidays, especially not in Muggle areas."

"Well, I tried out some simple spells after I got my wand, before coming to Hogwarts," Hermione replied. "Everything worked for me, but maybe you just have to be careful not to get noticed?"

"There's no way Gran won't notice," Neville gloomed.

Harry frowned. "I think I remember reading in one of the Wizard novels I got about something called the Trace. They can detect under-age magic, I think? Maybe they didn't stop you before you came to Hogwarts because they thought it was still accidental."

"Well… that hardly seems fair," Hermione huffed.

"I bet they have some kind of special exemption for the rich kids," Dean suggested. "So maybe-"

"Checkmate!" Ron called, interrupting them. "Good game, thanks for playing!"

The white king somehow managed to look contemptuous as he threw his crown to Ron's feet.

"That was fun," Ron added. "Is that Erised mirror thing through that door on the far side?"

Harry checked quickly, locating the scent, then nodded.

"Okay," Ron went on, and opened the door.

He shut the door.

"Guys," he whispered. "There's a really big troll in there."

"Another troll got into the castle?" Neville asked. "Right here where a student could run into it?"

"I think Hogwarts has a troll problem," Dean muttered. "What do we do?"

Hermione rolled up the sleeves of her robes. "Right!"

"Uh oh," Ron winced. "I know that look. That's how she looked when Professor Flitwick assigned us a spell she already knew how to do."

"I've spent a lot of the last two months reading about how to stop trolls," Hermione told them. "I have about six different plans for it, but two of them need Harry's help. Harry?"

"Of course I'll help," Harry agreed. "We want to make sure the troll can't hurt anyone."

Five minutes later, Harry was flexing his wing as they walked down the corridor.

"Is that all right?" Hermione asked.

"I said I was sorry," Ron mumbled, and Harry tapped his friend with the other wing.

"Don't be sorry," he told Ron. "That was brilliant."

"Yeah, I never would have thought of dropping the troll's club on its head," Dean nodded. "It's a pity the troll fell on Harry, though."

Harry shrugged. "It just stings a bit, that's all. But I was really impressed with that massive light spell, too… it's a pity these trolls don't turn to stone like the ones in The Hobbit. Maybe it's an Olog-Hai."

"A what?" Ron asked.

"It's like a troll that can walk around in daytime, made by the Dark Lord," Neville said, then went pink. "I mean, um, a different Dark Lord to that one."

"You've been reading the Lord of the Rings?" Hermione asked Neville.

Dean snorted. "I was there when Harry told him about the Ents. He was sold on it."

The conversation was interrupted by a sudden whump of flames lighting up, sealing off the way they'd come and the way ahead. One of the curtains of flame was an odd, sooty black, while the other was purple, and all five Gryffindors looked around in surprise.

"What now?" Ron asked.

"Oh, look!" Hermione said, sounding eager. "This is a logic puzzle!"

"First a troll and now a maths problem," Dean sniggered. "This is Hermione's lucky day."

"Logic and maths are different, though," Hermione muttered, mostly focused on the instructions and on the potions bottles on the wall. "Okay, so, um… wait, there's apparently poison in some of these?"

"That doesn't sound very safe," Harry mused. "Well, unless it doesn't really mean poison."

"Mate, to you nothing is poison," Ron shrugged.

"Okay, so… give me a minute," Hermione asked.

As Harry was examining the fire, wondering if anybody had something they could throw into it to see what happened, Hermione pointed. "Okay, so neither the biggest or the smallest bottles are poison, but the bottles second from the ends are both the same. So that means those two are the wine."

She moved them forwards a bit, then moved the two to their left back a bit. "And those two are poison… so there's one poison left and both the actual potions. One of the poisons is on the far left side, which means that the one opposite it – that one – is the potion to move back, because they're different and neither of them lets you move forwards."

Satisfied with her answer, she picked up the smallest bottle. "And this one is the potion to move onwards. There's not much of it, though."

Harry considered, then picked up the largest bottle and pulled the cork out with a squeaky pop!

"Harry, what are you doing?" Hermione asked.

"I'm testing if this is the wine," he answered. "It's the biggest one, so it's safe, right?"

"This should be interesting," Dean muttered, as Harry took a sip.

"I think that's wine, yeah," he agreed. "It tastes like wine smells."

On seeing Dean looking puzzled, he shrugged. "I don't know if I can get drunk, and I'm going to need to fly out of here – I'll find out some other time."

"So how do we all get over the flames?" Neville asked. "There's only one bottle and that's not going to be big enough for all of us."

"Or for us to get back," Hermione agreed.

They thought about it, then Dean pointed. "I know. Harry, you can have half of the bottle, and then you can lie down on top of the flames – that way you can block them and we can go over the top."

"I don't think this is the intended solution," Hermione frowned, passing the small potion bottle to Harry.

Harry was wondering if maybe the intended solution was a Bezoar, but he didn't have one with him. So he drank some of the bottle, which tasted sort of like ice, and lay down on the black flames.

The others crossed over, which was a bit awkward, then Harry followed behind them.

"Well, here it is," he said proudly. "Told you we could find it!"

Hermione took a look first, and blinked. "That's… that would be quite nice..."

"What is it?" Ron asked.

"I'm writing a textbook," Hermione told him. "No, my name is on the textbook… I think I'm teaching here. And… the class seems to have a three-headed dog in it, and at least two dragons."

"That sounds pretty cool," Dean chuckled. "Can I see?"

"That's not how the mirror works," Ron supplied, as Dean looked in it. "We tried."

Dean frowned. "I… okay, I didn't know that about myself."

At Harry's quizzical look, he flushed slightly. "It doesn't matter."

When Neville looked, on the other hand, he gasped.

"Is that a sword?"

"We can't see, Nev," Ron reminded him. "What's the sword doing?"

"I'm holding it in one hand and my wand in the other, and I'm fighting some evil wizards," Neville explained. "And – I think I'm keeping my Gran safe?"

They considered that.

"I think that's the most actiony thing any of us have had," Ron summarized. "Ever thought about using a sword?"

"No," Neville replied. "But now I am… I wonder if I could have one like that sword Aragorn uses."

After Harry had had most of the rest of the bottle, and they'd gone through the black flames again, and then all five of them had had some of the other potion and gone through the purple flames, past the troll, past the chess board (ignoring the rude gestures from the white chess pieces) and cleaned up the bluebell flames in the plant room, each of Harry's friends flew up with one of the brooms from the key room and then Harry took them and put them back before flying up and out of the hole himself.

"How did it go?" Fluffy's middle head asked. "Did you get on all right?"

"We got to see the mirror, so yeah," Harry agreed. "There's a lot of weird stuff down there though. You know there's actually a troll?"

"Oh, dear..." the left head sighed. "I did hear about your troubles with that troll that got into the castle last year, Hagrid was very distressed."

Fluffy carefully lowered the trap door back into place. "There we go. And do pop by again some time. It seems like half the students have at some point, though normally I simply roar at them and off they go."

"We'll have to visit some time," Harry agreed, pleased to have met another not-a-human who was such good conversation. "Can you read? I might ask Percy to enlarge a book for you."

"Hmm… we'd better not," said the left head, thinking about it. "It might undermine the menace slightly. But thank you so much for the offer."

Harry waved, pleased with how the afternoon had gone.

Then the door opened, and Professor Dumbledore came in.

When he saw the five young Gryffindors, he blinked. Then he asked Harry to move back a little to make room, and came in before shutting the door behind him.

"Good day to you all," he said politely, drawing up a chair – using his wand to make it out of thin air. "I must admit to being quite surprised to see you all – Miss Granger, Misters Weasley, Potter, Thomas, Longbottom."

"Good afternoon, Headmaster," Fluffy added.

"Is something wrong, Professor?" Harry asked, suddenly worried.

"Perhaps, perhaps, Mr. Potter," Professor Dumbledore said calmly. "Mr. Fluffy. Has anyone gone down the trapdoor?"

"Just these five," the right head said. "Said they were able to go through any doors that weren't locked."

"Well… we did sort of go through a locked door," Harry admitted. "But the key was right there, so it seemed more like a puzzle than anything."

"Dear me," Professor Dumbledore said, adjusting his glasses, and looked at the door. "I would have thought that my warning that the third-floor corridor on the right-hand side was out of bounds was adequate warning. And it was locked, I must add."

"It wasn't," Ron protested. "The door opened really-"

"We're on the third floor?" Hermione interrupted, going pale. "I didn't realize!"

Dumbledore chuckled. "I must admit that it is a little easy to get lost in good old Hogwarts… Mr. Fluffy, is Mr. Weasley correct that the door was not locked?"

"I fear he is," Fluffy's left head said. "People have been opening the door to look at me and then running away all last term. A few have done it so far today, as well."

"Dear me, it seems that the door really should be better locked," Professor Dumbledore mused. "Perhaps it is the problem with teaching a lock-opening spell here. Well, you mentioned the door with the flying keys… I assume then that you turned back at the chess set?"

"No, Ron got through that," Dean supplied.

"...oh, no, hold on," Neville groaned, sounding very much like he'd worked something out that he'd rather not. "This is all to try and stop people getting in, isn't it? The Devil's Snare is to catch anyone who gets past Fluffy, and the keys… I bet the troll was a guard, as well..."

"They're not very good at it, though," Harry said. "And why would you want to keep the Mirror away from people?"

"The Mirror of Erised is dangerous indeed, Mr. Potter," Professor Dumbledore said. "Many have spent all their time staring into it, wasting away and tantalized by visions of a world they cannot have."

"Mine didn't really look impossible," Dean said. "It'd just be a bit embarrassing to ask."

"Mr. Thomas?" Professor Dumbledore asked. "I believe you went back to your home for Christmas? Does that mean that you made your way all the way to the end, if you have seen the Mirror?"

"Well… yeah," Dean agreed. "Most of them weren't even that hard. Ron dropped the troll's own club on top of its head."

"We did use up all the flame freezing potions," Harry added. "Is that okay, Professor?"

Professor Dumbledore chuckled. "Perhaps… but what would you suggest, Mr. Thomas?"

"...make sure that only someone who's actually meant to be there can get past it," Dean went on. "That's the whole point, you put the dangerous stuff afterwards."

"My goodness," Professor Dumbledore said with a smile. "I fear I will be up for hours tonight making those changes; I may even need to be out of bounds after curfew, and I hope that Mr. Filch won't complain to the headmaster about it. What would you say is the most important?"

"Easy," Dean said. "Password on the door that only teachers get. Then for the potions room, just fill them all with… with… what was that special really powerful sleeping potion?"

"Draught of Living Death," Hermione supplied.

"Right, that," Dean agreed. "Nobody said the instructions had to lead you to the right bottle… you could just keep the right potions in your office or something, and if someone tries drinking it they'll just get frozen until you can go and look. Then there's all the other stuff, like making the Devil's Snare fireproof or something – oh! And there's one more thing I just thought of."

"What might that be, Mr. Thomas?" the Headmaster asked.

"Put the mirror in your office," Dean replied. "If someone tries to steal the mirror, and they can get all the way to the end, they'll be stuck there."

Professor Dumbledore stared for a moment, then laughed.

"An excellent idea!" he pronounced. "My, Mr. Thomas, you do have a quick wit indeed…"

"I think it's because of Monty Python," Dean said, which made Hermione stifle a giggle. Harry wasn't at all sure what she meant, and she told him she'd explain a bit later.

"So just to make sure I understand?" Fluffy asked with his middle head, raising a paw. "I'm to growl and try to scare off anyone who comes in, same as before?"

"Excepting myself and Hagrid only, yes," Professor Dumbledore replied. "And Minty, of course."

"Well, of course Minty!" Fluffy's left head agreed. "She's such a good cook!"

It was later than they expected when they made it back to the Gryffindor Common Room, and after a bit of discussion the five of them dispersed to get used to living back in Hogwarts again (or, for Ron and Harry himself, to get ready for the beginning of term).

The book club book had been longer than normal over the winter, and Harry decided he should give it a bit of a skim-read through his favourite bits. As he picked up the book from his trunk, though, something fell out of his robe pocket.

Harry examined the odd, red-coloured rock, shrugged, and stuffed it in the bottom of his trunk with the stuff he'd mentally labelled as 'snacks'.

AN:

In fairness to them all, this is a castle where you genuinely have doors that lead elsewhere on a Friday.

And yes, cerberi are not in Fantastic Beasts. So the natural conclusion is... Fluffy has the vote.