Harry stormed up the stairs, unable to bear their well-meaning platitudes and concerned glances one second longer. Where was all that when he'd been shut up at the Dursley's for weeks on end? Bit rich of them to sit there now, acting all shocked and worried, as if he was the unreasonable one! After he'd been left alone with nothing but his grief and the ruddy Dursleys for company for a month! He'd every right to be bloody angry.

He turned onto the landing, considering the corridor in front of him. He contemplated his and Ron's room for all of a second— no, he didn't want to be somewhere others could get to him. He didn't trust himself not to scream at anyone who came too close right now. But where could he go? He turned back to the staircase and kept climbing.

Though Mrs Weasley had had a year to get the house to rights, some of the rooms on the third floor were still untouched— there was already more than enough space for the Order's purposes, after all, and time was precious. They'd been banned from entering the remaining rooms, of course, but then, reasoned Harry, it was his bloody house! Sirius had left it to him. Why shouldn't he go wherever he pleased? And no one would think to look for him up here either. It could be a bit dangerous, theoretically, but what of that? He was marked for death anyway, thanks to the prophecy.

Harry continued down the corridor, not pulled in any one direction, waiting to see if any room in particular took his fancy. As he continued, he heard something. Or thought he did. He stopped, straining to listen, and stepped back a few paces, the floorboards creaking beneath him. Voices. Whispering voices. They reminded him of the Veil. Of Sirius. There wouldn't be a veil in Grimmauld Place though, surely?

His ear to the door, he could hear them. Murmurs, not fully audible. No recognisable words. Withdrawing his wand, he placed his hand hesitantly on the doorknob, and turned.

The room he entered was cluttered, coated in dust and grey in the half-light that filtered in through the thin, closed curtains. If Harry had to guess, he'd have said it was once a study— a large table or desk was against one wall, covered by a draped sheet to keep away dirt and scratches. There were boxes everywhere. Harry could see why this room had been left; not big enough to be of any real use, filled to the rafters with boxes and likely teeming with dark artefacts.

But his attention was drawn to an object, covered by a sheet, towards the left of the room. It reminded him immediately of the Mirror of Erised— the domed shape, the thin profile. He could hear the whispers from beneath the sheet that covered it, and fancied that he could see light around the edges, as if it were a window with a view to a sunny day beyond. A part of him advised him to leave it alone, to fetch someone, but he'd never been good at leaving mysteries unanswered, and found himself drawing closer, shifting boxes and furniture to clear a path.

In a few short minutes he stood in front of it, the noises louder here. No longer murmuring— he thought he could hear shuffling, the shifting of heavy items. The occasional word maybe- the voices sounded familiar. He reached out a hand and, with a smooth motion, dragged the sheet off the object, causing a plume of dust to rise into the air as he did so.

He pressed his hand to his mouth, covering the sharp intake of breath. In the mirror– for it seemed to be a mirror, though it did not show his reflection– he saw a room beyond, very similar to the one in which he now stood. But he was not there. Or rather, he was, but on his knees, some distance away from the mirror itself, sorting through boxes. Harry turned to look behind himself— no one was there. He turned again to look into the mirror, observing carefully.

It was him, no doubt about that, but it was also not him. His glasses were different– rectangular and black– lending a more mature countenance to his face overall. His hair was different too; longer, tied in a ponytail hanging down his back, no longer untamable, possibly due to its length. He was dressed differently— wearing robes, but not his Hogwarts robes or his formal dress robes. Something in-between; casual robes, like those worn by Remus, yet in a finer fabric. Dark green and well-made, arranged neatly around his legs as he knelt on the floor, up to his elbows in a box marked, 'books'.

"Harry?" came a voice from the mirror, faint but easily distinguishable as his name.

"In here!" called Mirror-Harry, without pausing in his perusal of the book he'd most recently lifted from his box. Harry turned his attention to the open doorway behind his alternate self, wanting to see who emerged, before cursing himself silently for his idiocy— what if the mirror people could see him, too? Who were they? Were they an illusion, created by his own mind, like in the Mirror of Erised? Or something sentient— dangerous? He moved to the side, out of the immediate eyeline of the mirror, and, crouching low, situated himself between two boxes, looking through the gap between them at the mirror beyond.

Behind Mirror-Harry, a tall dark shape appeared, black robes billowing around its legs and contrasting sharply with the pale face above them, framed by limp black curtains of hair. Snape! Or Mirror-Snape, Harry supposed, glancing behind himself again, seeking reassurance that the horrifying apparition was not, in fact, standing in the room with Harry.

Harry returned his attention to the mirror once more, as Mirror-Snape cast his eye appraisingly around the room he had entered before fixing his gaze on Mirror-Harry. Harry did a double-take at the expression on the man's face. There was no look of hatred, no sneer of disdain. The look on his face was relaxed, conveying an emotion that a disinterested onlooker might mistake for fondness.

"Have you found anything of interest?" the man enquired mildly, and Harry was once again struck by the lack of venom, this time in the man's tone of voice. Since the very first register the man had taken in the very first potions class of Harry's first year, Snape had always spoken to him with thinly-veiled contempt, at best, and obvious loathing at worst. This Snape seemed almost friendly in manner, and it was so incongruous with Harry's experience of the man so far that he became convinced that this must be an image created by the mirror, and it hadn't gotten it even remotely right.

"Are you asking me to judge books by their covers?" asked Mirror-Harry, and it was utterly bizarre to hear his voice coming out of that... that impression of him. It was, Harry imagined, akin to how Aunt Petunia always exclaimed over how strange her voice sounded on video. Harry had no personal experience, because of course they'd never bothered to capture him on film.

Mirror-Snape snorted –snorted!– and offered a mug to Mirror-Harry, who brushed his hands together to rid them of dust before accepting it. Snape cast an expectant look at Harry's counterpart.

"Well, as we thought, they're mainly dark-arts," expounded Harry's mirror-self, with a grin, "written in a variety of languages, including one or two I don't even recognise, never mind speak. It'll be worth going through them when we get back; there might be something useful. I think this one looks promising," he grabbed one from the top of the pile next to him and handed it over to Snape, who regarded it with interest, flicking open the front cover, his eyes shooting across the page as Mirror-Harry sipped on whatever was in the cup.

It looked quite companionable, and Harry couldn't get over the unlikeliness of the scene. And what had his mirror-self meant by, 'when we get back?' Where would they be returning to together? Hogwarts? Why would Snape go with him on an outing from Hogwarts?

"What have you got the house elf doing?" asked Mirror-Harry, in between sips.

Mirror-Snape answered without looking up, "General disinfection. The thing is clearly mad, of course, so I've little faith that its efforts will make any discernible impact on the filth." He raised his head to meet the gaze of Mirror-Harry. "Once we've cleared everything of danger and value, it might be worth paying someone to come and sort out the rest."

Mirror-Harry hummed thoughtfully. "I wondered about Lupin."

"The wolf?" Snape sneered. This, at least, thought Harry, was a believable reaction.

"Do we know any other Lupins? I thought maybe he could come and take this place on as a project. I could pay him for it and it'd give him a place to live."

Snape seemed to bite back his gut response and paused, thoughtfully. "The idea has merit," he intoned, slowly. "You know my opinion of the wolf, but he is competent enough to deal with the... unique charms of your newly-acquired property, and considers himself honest enough not to take what is rightfully yours. You may, however, find that he is inclined to keep from you anything he feels is too dangerous for you to handle, and he is likely to pass any such objects on to the headmaster, who may or may not deign to inform you of their existence."

Mirror-Harry frowned. "Could I write something into a contract to prevent that?"

"Potentially. As you know, legal matters are not my forte— we would need to seek advice. Now," he cast his eyes around once more, "would you like to continue in this room?"

Mirror-Harry nodded, "I've still got these books to work through. You could start over there?" He gestured in the vague direction of the back of the room. Towards the mirror.

Harry held his breath as Mirror-Snape worked his way around the room beyond the mirror, mind in a whirl thinking of all that he had overheard. Newly-acquired property.... Had Mirror-Harry also inherited Grimmauld Place? And why was Mirror-Harry seeking Snape's advice? Referring to Remus as Lupin? Why did Snape refer to Harry by his given name? The smooth baritone of Snape's voice drew Harry's attention away from his thoughts and back to the mirror.

"This is an intriguing object."

"The mirror?" came Mirror-Harry's absent-minded query, his nose in yet another book.

"Indeed. Though I don't believe it is a mirror, per se."

"No?"

"No. It doesn't reflect— it is more of a window than a mirror."

Harry could see where Mirror-Snape stood, large as life now he was up close to the mirror's other side, clearly perusing the details around the edge. Mirror-Harry came to stand alongside him, and Harry was surprised to see that his mirror self was only slightly shorter than Snape. Significantly taller than Harry himself.

Mirror-Harry read aloud, "Fenestra ad alium mundum- window to another world. Have you heard of this before?"

"In passing," replied Snape, "The window of Alium, it is said, shows a reality that is different to our own in one key way. Its use is to answer questions- the sort of, 'What ifs' that plague those who suffer with a proclivity for introspection."

"Oh." Mirror-Harry (or Alium-Harry, if Alium-Snape was to be believed,) moved to peer more closely into the artefact, but Alium-Snape caught his arm and pulled him back sharply. "Have I taught you nothing, Potter? NEVER touch a potentially dark object. For all you know, you might be trapped in there forever."

Alium-Harry flushed slightly, muttering his apologies as Snape softened his expression before returning his attention to the Alium.

"What do you see, Harry?" Snape asked, speaking in a softer tone than the scolding bark of a moment ago. Alium-Harry peered closer, this time careful not to lean too far.

"It looks like a mirror image of this room, more or less, but without you and I." His voice slowed as he deliberated, cataloguing the differences, "The box of books is still closed. There has been a disturbance though, like someone recently cleared a path to the mirror... Do you think we're being watched?"

"Very good," remarked Snape, a note of genuine approval in his voice that Harry had only heard directed at Malfoy, and then only rarely. "Care to conjecture as to the identity of our observer? We should be safe from here, but to be certain..." Snape raised his wand and muttered under his breath, causing a wall of blue to flash momentarily across the mirror, or window, if alternate Snape was to be believed. "That should prevent ingress of spells or matter."

"If I had to guess," said Alium-Harry, somewhat hesitantly, "I'd say a member of the Black family, or maybe even my alternate self. Or yours." Snape motioned with his hand for Harry's counterpart to elaborate. The other Harry continued, "This is the ancestral home of the Black family, which makes one of them a possibility, if they're not all dead in this alternate world. But then," he paused, thinking if over, "from what you said about the mirror, it deals in what-ifs, so maybe it's your alternate self, or mine."

"I'd say it's most likely to be yours," opined Snape, "Given that whoever it is has left a very obvious trail and chosen a rather conspicuous hiding place." At this, a pair of obsidian eyes locked on to Harry's with precision. "Come out then, Mr Potter. I promise you, we mean no harm."