The room after the one adjoining the chess room (which had revealed nothing of interest after the Fiendfyre bird had flown through) contained Potions equipment and ingredients, but Professor Quirrell produced a flask from his robes and poured it over the flames that had sprung up in the opposite doorway. He flicked his wand, and a number of ingredients vanished.

A wall of solid blackness had appeared in the doorway.

"Mr. Potter, when I dispel this barrier, you must remain beneath the True Cloak of Invisibility until I say so. That is a categorical order. Do you understand?"

Harry nodded, taking the Cloak from his pouch and pulling it over his shoulders.

"Ansswer in Parsseltongue."

"Sshall remain beneath Cloak until otherwisse ordered."

Professor Quirrell stooped down and pulled the corner of the Cloak over himself, standing as far away from Harry as possible. The sense of doom was more bearable now that Harry knew its provenance, but it was still uncomfortable.

Harry looked incredulously at Professor Quirrell. "Two people can wear the Cloak at once?!"

"It would appear so, yes. You already knew that it was not necessary for the Cloak to cover your entire body, did you not?"

Harry smacked the side of his own head, none too gently. That had been rather a lot of wasted effort in Azkaban.

The black barrier melted away.

The mirror in the centre of the room looked less like a mirror and more like a portal to a copy of the rooms behind Harry. It was framed in ornate gold, with wrought golden feet. The mirror did not look balanced, nor did it appear fixed to the floor. It was simply there, more solid by far than the walls and floor surrounding it, like a singular fixed and determined point relative to Earth's motion, no more moveable than the universe itself.

The Defence Professor gestured Harry around to the back of the mirror, which was plain gold, then hissed "It iss ssafe."

Harry stowed away the Cloak.

"This, Mr. Potter," said Professor Quirrell, taking on once more the tones of a lecturer, "is the last known surviving relic of Atlantis. Merlin called it the Mirror of Erised; Olga Xenda, supposed tutor of Baba Yaga, named it the Mirror of Vec; some scholars name it the Mirror of Noitilov; and a few scattered ancient records refer to it simply as Nillits."

"I take it you can't brute-force this one, Professor?"

Professor Quirrell smirked. "It is said that the Mirror of Noitilov perfectly reflects itself. It is the safest, most secure object in existence, having survived the Flood of Atlantis that wiped our forebears out of Time. It can be used to hide and trap objects and people, and it is where Dumbledore has hidden the Philosopher's Stone. No, Mr. Potter, I rather doubt Fiendfyre will have much of an effect upon it."

"What does it actually do?"

"It has perfect, unchallengeable power over all that it reflects, and can create alternate worlds, though only those as large as what it can reflect. Its power is supposedly based on the innermost feelings and desires of its viewer."

That didn't sound like a very satisfying explanation, and Harry said so.

The head tilted. "Hmm. Tell me, student of Muggle arts, what do the runes near the top of the Mirror say?"

The runes were simple, bold black lines and dots, curving gracefully and coming to abrupt, straight stops, looking quite unlike any writing system Harry had ever seen. Harry had had trouble enough with Latin. "I'm sorry, Professor, I don't recognise-"

"Read them anyway. Not dangerouss."

Harry looked once more at the runes, and opened his mouth. "Noitilov detalo partxe tnere hoc ruoy tu becafruoy ton wohsi." Harry looked away, blinking. That first rune was noitilov, and it meant what you detalo partxe so that it was tnere hoc -

It was almost impossible to describe: the runes didn't seem to relate to any other concepts, didn't form words in his head... Harry couldn't visualise the runes when he looked away, let alone transliterate them...

Harry shook his head and reinforced his Occlumency barriers, but felt the same effect.

"I take it you do not understand, then? A pity."

Harry had a sudden thought. "Voice recorder," he said to his pouch. Muggle technology didn't work very well at all in Hogwarts, as Harry's experiments had shown, but the pouch seemed to block or escape the magical field, and something this simple would take at least a few minutes to be severely affected - especially with Harry's own additions.

He read the runes aloud once more.

Harry turned his back on the Mirror and took a Quotes Quill from his pouch, then pressed the "play" button. The Quill scratched mechanically over a scrap of parchment.

Noitilov detalo partxe tnere hoc ruoy tu becafruoy ton wohsi.

"Um... 'I show not your face, but your coherent extrapolated volition.'"

Professor Quirrell was staring at him.

And then he began to laugh.

Harry had seen Professor Quirrell smile before, and give short, sardonic chuckles, but this was wild, genuine laughter, almost like Dumbledore had done when Harry had blackmailed him. Harry had never once expected to see the dignified and mysterious Defence Professor double over, clutching at his sides, but there he was.

Professor Quirrell collected himself with visible effort. "Ah, Mr. Potter," another chuckle, "I was about to say that it is known that even the greatest artefact can be defeated by a lesser, yet specialised counter-artefact," another burst of laughter, "and yet I never expected the last legacy of Atlantis to be brought low by a Muggle cassette tape."

Harry felt he was entirely justified in grinning smugly. Modern Muggles one, ancient Atlanteans zero.

"Well. That alone was worth the visit, and reaffirms what I suspected. The Mirror is a device intended to grant wishes, and to prevent the end of the world in such a way by showing a coherent extrapolation of the user's desire. Perhaps the Mirror transports those reflected therein to alternative universes... Or perhaps it simply traps people and objects in a point of time, as some tales describe. Now, Mr. Potter, to the task of retrieving the Stone."

The Defence Professor rolled up his sleeves, flicked his wand and said "Kulyok." A translucent, silvery-looking pouch appeared in the palm of his hand. "This pouc- pardon me, this pouch should collect anything the Mirror disgorges, and block any curse or contact poison applied." Quirrell grinned. "You see, there is a rather nasty potion known as Bahl's Stupefaction, of which Alastor Moody is fond. When contacted or imbibed, it has interesting effects on the cunning. The Dark Lady who called herself Lethae, dosed with such a potion, once kidnapped and attempted to interrogate one of her enemies in exchange for the lives of his friends. Her enemy was known to be clever and underhanded, so she confiscated all of his belongings down to the clothes on his back, abducted him to a graveyard of all places, went so far as to force him to make an Unbreakable Vow, made vivid and terrifying threats against all he loved, and raised wards against any conceivable intervention. Unfortunately, due to the Stupefaction she neglected to take his wand, and he shot her whilst she was busy monologuing."

Harry made a mental note to see if he could get Fred and George to slip some of that stuff into Draco's cornflakes.

Hidden beneath the Cloak, the two circled round to confront the Mirror of Erised.

OoOoO

The Mirror of Noitilov stood, solidly anchored - although, in fact, it looked more like the Mirror was anchoring the rest of the Universe. If Harry hadn't known that the Earth was in fact moving through space at enormous speeds, and more to the point that all motion was relative, he might have thought that all reality was centred on the Mirror.

"Do you have any ideas, Mr. Potter?"

Harry thought.

"Why is the Stone in the Mirror in the first place, Professor?" It would make some sense, and be exactly Dumbledore's style, to set up a vast system of incredibly easy traps guarding a Mirror that could show the viewer's heart's desire (which could be an excellent motivator for a student) and just so happened to also be the last legacy of Atlantis, the obvious and most secure hiding place for anything, and then actually keep the Philosopher's Stone in his sock drawer. Or, in fact, his pocket, so the would-be thief would have to duel Dumbledore...

"Do you know the history of Baba Yaga, child?"

"Only vaguely." After the school's reaction to her name at the beginning of the year, Harry had looked up the "undying" witch, and found standard fairy-tale fare. She had supposedly been a shapeshifting, immortal Dark Witch who had lived for centuries and vanished some time after teaching Battle Magic at Hogwarts. It was said that she devoured naughty children, which was used as a threat by parents of children too young to wonder what possible motive an immortal Dark Lady would have to enforce childhood discipline with cannibalism.

"Baba Yaga lived far longer than any other recorded witch or wizard, and there is evidence of her shifting forms at will, though she was never said to be a Metamorphmagus in her youth. She held the Stone of Permanence, obviously."

Actually, there was something more important...

"Professor, sorry to interrupt, but how is the Stone made? I saw an alchemical recipe-"

"A lie. Simple misdirection, intended to frame possession of the Stone as some earned right, to soften the blow." He scowled. "Magic is not permanent, as a general rule. One of the greatest feats of Merlin himself was the permanent Conjuration of the Most Ancient Hall of the Wizengamot, and he made no habit of such magic."

Professor Quirrell gave Harry a calculating look.

"The Stone was clearly intended as a healing device. In addition to its primary power, it can perform with a touch feats of healing beyond Muggles or wizards, delicate and obscenely complex and powerful healing magic. The Stone's abilities are eldritch even by my standards. Therefore, the Stone must be unique, and very old indeed."

"Oh." That was objectively the worst news Harry had heard in quite a while. He had entertained the prospect of some sort of mass-manufacture of Philosopher's Stones to provide immortality to the masses, and before learning what the Stone did, he'd even briefly wondered if anyone had ever actually tried to turn the Atlantic into Elixir of Life.

"Disappointing, I quite agree. Still, what magic has made, magic may yet make again. Regardless, some six centuries ago, Baba Yaga taught at Hogwarts. Lest she do any harm to the students or faculty, or they to her, an ancient device called the Goblet of Fire was used: Baba Yaga would spill none of the students' or faculty's blood, and take nothing of theirs, and they would extend her the same courtesy."

Professor Quirrell looked speculatively at the ornate edges of the Mirror.

"In her sixth year of Hogwarts was a beautiful, clever, horrifically evil witch named Perenelle. She seduced the Dark Lady over the months, and exhorted her to use her power of transformation to take the form of a man. The Goblet counted what followed as the shedding of Perenelle's blood and the taking of her virginity, and so Perenelle murdered the forsworn Dark Witch, and took for herself the Stone."

Professor Quirrell paused. "Of course, one does not survive six centuries without achieving at least a little cunning. Perhaps the entire story was concocted such that Baba Yaga could start a new life. It is entirely possible that that story, which took quite some effort to uncover, is merely another layer of misdirection, and that "Nicholas and Perenelle Flamel" are really Baba Yaga and Perenelle pretending to be Perenelle pretending to be both herself and her husband, and beyond that I cannot predict at what level she or they play."

"And the Mirror?"

"Ah, yes." Quirrell withdrew from his robes a golden sceptre, which should have been too large to fit. "I arranged for an inscription to be uncovered, claiming that this device could track the Stone wherever it lay, then carried out an ostentatious theft bearing the mark of Voldemort's hand. I believe the Stone's holder, whom I shall call Perenelle for the sake of brevity, insisted that the Mirror be used, which alone might evade even the greatest scrying."

"So Dumbledore hid it in the Mirror. What if only he can retrieve it?"

"The Mirror must be fair. It cannot be set to distinguish an individual. It can act only upon its viewer's hopes and dreams."

"How about, 'the individual must know that the password is- ' wait, no, it has to involve the viewer's wishes... Could the Mirror be set to only respond if the viewer's wishes perfectly align with Dumbledore's?"

Quirrell tapped his cheek. "Possibly. It seems unlikely, however, that the Mirror would be so exclusive..."

"Could you Confund yourself into being like Dumbledore?"

"Even if I could do so convincingly, the Mirror sees through such lesser effects. It is claimed that even Obliviation will not fool the Mirror. It would be utterly impossible to trick the Mirror by simply assuming Dumbledore's persona."

"Well, in what circumstances would Dumbledore want the Stone to be retrievable?

Quirrell looked as though he wanted to start pacing. "Mere circumstances will not matter to the Mirror - I begin to worry that this puzzle is insoluble. The Mirror must respond to a genuine volition, a true and heartfelt aspect of the viewer's real wishes, hopes, dreams, intentions... Ah. Ah."

The Defence Professor smiled unnervingly. He stepped out from under the Cloak and held out the translucent pouch in his left hand, gazing directly into the Mirror.

In the pouch, visible as though nothing were surrounding it, was a small chunk of shining scarlet stone, smooth and glassy and irregular. The pouch's appearance did not change upon contact with the Stone, but it somehow seemed more decisively real, fixed and solid.

That's not fair, complained Ravenclaw. How did he-

Then the Professor vanished, but his reflection in the Mirror remained, alongside another one.

"Hello, David," said Albus Dumbledore.

Author's Note:

Seemss to me that ssubreddit'ss collective intelligencce sshould be ssufficcient to guess meanss of taking Sstone from Mirror with cluess given. Yess, Parsseltongue hass word for "ssubreddit".