They didn't need to wait long. Downstairs in the kitchen of Grimmauld Place, the meeting was disrupted rather efficiently by the dramatic arrival of a patronus deer, which walked up to Severus and seemed to nuzzle at his ear before vanishing into a puff of ether.

"Severus?" The room was entirely silent, save for the headmaster's query, the tone of which underscored its urgency.

"I do not know where the patronus originated," responded Severus. "It used my voice, and said only, 'Find Harry.' I presume it refers to Potter- where is the boy?"

It was Lupin who spoke, "Upstairs somewhere, I think; his morning was somewhat fraught, and we thought he might benefit from some time alone."

"Molly," began the headmaster, "Would you please go and satisfy our minds as to Harry's whereabouts and wellbeing, so that we may continue?"

The panic began when none of the children loitering around and pretending not to eavesdrop outside in the hallway had seen Harry since his earlier outburst. The panic intensified when it emerged that he was not in any of the downstairs rooms, the study or his bedroom. The panic reached fever-pitch when a search of the fourth floor revealed a glowing blue door that resisted all attempts to force it open and delivered a rather nasty shock to anyone who tried. The panic seemed to solidify and become tangible when the headmaster's whispered, "Point me, Harry Potter," resulted in his wand aiming directly at said blue, glowing door.

Mrs Weasley was beside herself by this point, and she wrung her hands as she spoke. "Oh, what does he think he's doing in there all by himself? I've told them all not to go near these rooms! We should have cleared them regardless, I knew we should have! What if he's hurt- can't you do something Albus?"

"Calm yourself, dear Molly," said the headmaster absently, casting a diagnostic charm over the ward. "What is your impression, William?"

The eldest Weasley son, busy working his own magic, whistled. "It's a complex one. Old too- I can get through it, I think, but it'll take some time. I'd love to get a look at the thing that triggered it."

"Severus," called the headmaster, drawing the Slytherin head of house rather reluctantly away from the spot he'd found by the wall, observing everything but interacting with little, "Could you come here for a moment?"

As Severus stepped closer to the door, the blue glow dimmed noticeably. "As I thought," mused Dumbledore. "Could you try to touch the door-handle, my boy?"

Bracing himself for the likely shock, Severus did so, and found that he could touch the door handle without suffering ill-effects. A number of attempts were made by others to piggy-back along with him, but touching the door or the man resulted in the aforementioned nasty shock. After a moment, it was reasoned that, for some reason, only Snape could pass the wards.

"Severus," said Dumbledore, beckoning him closer, "I'll need you to keep whatever is holding Harry in there distracted for long enough that we can get through the wards. By all means, if you get the opportunity to leave with him, try to do so, but I do not anticipate that you will be capable of leaving once you have entered."

Snape nodded once; he, too, had deduced that much.

"Are you prepared then, my boy?" asked Albus, urgently, and in response, Severus returned to the door handle, tried to grasp it, and noted that his hand immediately passed through it. At a confirmatory nod from the old man, he steadily walked forward, propelling himself through the door itself and into the room beyond.

************************************************

Barely ten minutes after the patronus was cast, the door at the end of the room rippled and Snape appeared, his face impassive, wand drawn. He seemed to walk straight through the door, which solidified behind him as soon as he had cleared the threshold. He glared assessingly at the sight that met his eyes.

"Hello, sir," Harry broke the silence, his voice artificially cheerful.

"Potter, you absolute imbecile," he spat, his voice cold and dangerous, "Come away from that object, immediately!"

Harry looked uncertainly at his friends in the mirror. "Go on, Mr Potter," encouraged Alium-Snape, his deep voice calm and soothing, forming a sharp contrast to his own Snape's hateful bark of command. Harry opened his mouth as if to protest, but then closed it again and did as he'd been bidden, picking his way across the room reluctantly to stand beside his surly potions master. Snape dropped his gaze to Harry for a short moment, giving a quick once-over from top to toe, as if assessing for injury, before he barked out his next order, "Try the door."

"No! We shouldn't leave yet; you need to talk to them! They won't hurt us!" protested Harry, but at a firm glare from his hated professor, he turned and approached the door, to find that the door was completely solid and the handle wouldn't budge.

"It's locked, Professor," he called out.

"Then unlock it!" Snape gritted out.

"The last time I used magic outside of school, I was tried by the Wizengamot!" Harry countered, aggressively.

"Unless you want me to demonstrate some of the things that I have been tried by the Wizengamot for, Potter," Snape snarled, his voice dripping with disdain, "Open the door!"

Trusting to the Fidelius, Harry tried, "Alohomora!" The spell produced a flash of light, illuminating the doorknob, but when Harry jiggled the handle, it still refused to turn.

"Now," came Snape's voice, distantly, and Harry turned to see that it had come from the Alium, where Snape's alter ego stood, arms folded across his chest, eyebrow arched and his trademarked sneer marring his face, "Are we quite finished with our foolish wand waving and silly incantations?"

Harry couldn't help it. He snickered, along with his alternate self. Snape ignored the comment entirely.

"I'm glad you find this amusing, Potter!" he hissed. "Your usual unbounded arrogance and sheer idiocy has managed to once more land you in peril, and myself into the bargain. Most diverting, I am sure. Are explicit instructions not to go into the uncleared rooms insufficient for the great Boy-Who-Lived, or do you simply delight in the drama?" He sneered, "Perhaps you've not been receiving the expected level of adoration since your last bout of Gryffindor stupidity?"

Glancing back at the Alium, Harry noted that his alter-self was frozen, a look of disgust warring with horror on his face as he observed Snape's vitriol. Conversely, Alium-Snape remained in his earlier position, eyes impassive but dark. Bolstered by the knowledge that the Alium duo were less than pleased by Snape's horrid display, Harry decided to take the high road and ignored the jibes. "Your alternate self, sir," he managed with a decent amount of dignity, as he ignored the rage that had begun to bubble through his veins- really? Arrogant? Him? "calls it the Alium."

At this, Snape whirled around and raised his wand towards the Allium, directing his next question at his alternate self.

"You're certain?" he barked.

"Reasonably," returned Alium-Snape. "I believe your Mr Potter here was the one to activate it, so the Alium will keep us all here until such a time as it determines that he is satisfied that he has learned all he wishes from us. Harry and I," here he jerked his head towards his version of Harry, "Have answered most of his questions. I thought it prudent, however, to send for you, as I am in possession of information that I believe may prove highly useful to you."

At this, Snape arched his eyebrows and muttered under his breath, drawing complicated little shapes with his wand. There were a variety of flashes and sparks, and when he had finished, he at last lowered his wand and used his other hand to pinch the bridge of his nose as he closed his eyes in apparent frustration at the results of whatever spell or diagnostic he had cast. He uttered something to himself that definitely included the word, "Potter!" before giving a long exhale and drawing himself up to his full height.

"Very well then," he announced, his voice clipped, "about what did you wish to speak?"

************************************************

The alternate Snape cast his eye over his counterpart. They were incredibly similar, though the Alium version flattered himself that he looked better groomed, and generally happier. "You are aware of the properties of the Alium?" he asked.

"Obviously," returned Snape, cooly. "If this is indeed the Alium, I presume that you are from a similar yet different reality?"

The other Snape nodded his agreement. "Precisely. We have discerned that the root change is that, in our reality, Harry did not talk the sorting hat out of placing him in Slytherin."

At this, Snape turned to stare at Harry. "You could have been sorted into Slytherin?"

Harry smiled mirthlessly. "Surprise? Apparently, this," he gestured at the mirror, "would be the result if I had let it happen." He sighed, "You-Know-Who isn't back in their world. Seems like I'd have done everyone a favour if I'd listened to the stupid Hat after all."

Alium-Snape frowned. "You've never been one for self-pity, Harry, don't start now. If you would, please tell your teacher everything you've learned about horcruxes today. He is likely to find it highly relevant, and I would like to reassure myself that you've grasped it fully."

Harry nodded once, turning to face the sneering visage of his potions master, swallowing his nerves. "In their world, Vold- I mean You-Know-Who, sorry professor, made seven horcruxes, which are pieces of his soul trapped in an object. The diary I destroyed in second year was one, and they can be destroyed with fiendfyre or basilisk venom. They," he gestured at the Alium, "told me that one is with Kreacher in Grimmauld Place,"

"Slytherin's locket," interjected Alium-Snape, with a nod.

"Right," said Harry. "And there's Hufflepuff's cup in Bellatrix Lestrange's Gringott's vault, Marvolo Gaunt's ring,"

"In the Gaunt Shack," added the alternate Harry, helpfully.

"Indeed," intoned Alium-Snape, "Be warned though; there is a fatal curse embedded within the ring. It should not be worn under any circumstances."

Harry thought he saw his professor twitch slightly at that, but the moment passed so quickly he wouldn't like to swear to it. He continued, "Umm, one at Hogwarts, what was that?"

"Ravenclaw's diadem," supplied Alium-Harry. It's in the Room of Lost Things, within the Room of Requirement."

Harry nodded his thanks and continued, "You said Nagini was one and that," here he took a fortifying breath, glancing at Snape worriedly before continuing, "That I'm the last, though unintentional."

Snape stared at his Harry, abject horror written on his face. Well, barely hinted at, but with Snape that was practically the same thing.

"Your scar," corrected Alium-Snape. "Not you. That's the main avenue we're looking down for a solution to that particular problem."

"He said," continued Harry, ignoring the incredibly disconcerting look on his Potions master's face, "That because my horcrux was unintentional, it probably isn't as tightly bound to me as the others, so may not require my destruction to remove. They're looking for a solution in these," he gestured to the boxes around him.

"Apparently, we have to destroy the horcruxes if we want killing You-Know-Who to stick. They're doing much better than we are, because he," Harry jerked his head towards the Alium, "wasn't an idiot and didn't ruin Dumbledore's plan back in first year."

"Because," came Alium-Snape's voice, with a tone that came closer to biting than anything he'd heard from Snape's alternate self so far, "My Harry had a trusted adult who told him the truth when he needed to hear it. You have not been an idiot, Mr Potter- you have, somewhat predictably, stumbled around in the dark and made some rather unfortunate choices as a result of that. Do not blame yourself."

Snape made a derisive noise at this, drawing the attention of his alter-ego.

"Professor Snape," the other Severus said, his voice carefully neutral, "I would like to talk to you about some observations I have made that I would prefer not be overheard by our respective Harrys. Could you come closer and cast a muffling charm? I shall do the same on my end."

Snape cast a suspicious look at the artefact, but with a sharp, "Stay here, Potter," swept over to it, using an unfamiliar wand motion that filled the air with a soft buzzing. Harry could no longer hear anything from the Alium.