Snape rubbed his face with one hand, using the other to put the book he'd been reading down on the desk and push it away.

It was almost noon, and he'd been up since shortly before dawn, researching the various uses of Dementor's blood. It was very hard going, even with his knowledge and experience, and he was aware that his present mood would not lead to a pleasant lesson for Potter.

Not that his job was to make things pleasant for the boy - or, indeed, for any of the students - however, there was no time to waste during these lessons. He had to teach Potter as much as he possibly could over the next month.

And I can't even blame Dumbledore for this, as I went ahead and decided to do it of my own free will, he thought sourly. He was beginning to develop a headache above his right eye - a headache which he knew from long experience wouldn't respond to painkilling potions.

He needed to eat. Part of the reason for the developing headache was the fact that he hadn't had breakfast this morning; he'd been too immersed in his research, and aware that it was best done on an empty stomach. As he'd said to Potter, not all Dark Witches and Wizards were evil; but those who weren't never used Dementor's blood. If they wanted to induce despair, or some other effect of the Dementors, they simply found other means with which to do so.

Which meant that for the past seven or so hours, he'd been reading some of the foulest Dark Arts books in existence - the other reason for his headache. The taint of twisted magic clung to them, assaulting him even through his shielding.

Calling one of the house-elves, he ordered a substantial - for him - lunch, and told it to inform Potter that his lesson would be put off until two o'clock. That should, with any luck, give him enough of a chance to regain his self-possession.

"Of course, Professor Snape, sir. Melly is being very pleased to help," the house-elf replied, and disappeared, reappearing a minute later with a full tray. "Dobby is giving Harry Potter the message, sir."

"Thank you," Snape replied, forcing the courtesy past the pounding in his head. "And please express my thanks to Dobby as well."

"Melly will be pleased to be doing that, Professor Snape, sir," the house-elf replied as she placed the tray on the only clear area of his desk, and disappeared again.

Snape waved his wand and sent the book flying into a large, shielded safe-box on the far side of the room; another wave of his wand closed it and a bit of the tension he felt eased as the worst of the taint was safely locked within.

Returning his attention to his desk, he placed the notes he'd made on his research into the warded drawer, and began eating.

An hour later, feeling somewhat better - though he still had the headache, the pain had reduced to a manageable level - he finished copying what Potter would need from the book on the Sensitive's gift, and closed it. Potter was due in almost an hour, which gave him more than enough time to write a note to Mr. Weasley concerning the wards - he'd had an idea in the middle of his copying - and take it to the Owlery to send it off.

The walk stretched his legs, easing the headache another notch, and he was leaning against the Owlery wall, looking out at the grounds and enjoying the fresh air coming in through the open windows, when he saw a flicker of movement on the far side of the lake - just beyond the wards.

Straightening up, Snape frowned as he focused on the area where he'd noticed the movement, straining to make out details.

The movement came again, and this time he was able to identify it as being a robe blowing in the breeze - which meant that there was an unknown witch or wizard out there.

Snape's frown deepened. No one should be that close to Hogwarts without informing the headmaster, even in the best of times - which these were most definitely not. Which, in turn, meant that it needed to be investigated.

Gesturing to himself with his wand, he murmured, "Inter Lateo," - the Notice-Me-Not Spell, one of the more useful spells in his repertoire when it came to dealing with both spying and students breaking curfew - then turned and began hurrying toward the school's main entrance. His firm stride was much quicker than running would have been - the castle had an unfortunate habit of taking people who ran through its corridors far out of their way. If he was at all lucky, whatever was going on would keep the person - or people - involved occupied until he arrived.

It took him almost ten minutes to reach the far side of the lake from the school itself, and by the time he arrived, it was evident that they were preparing to depart.

Lucius Malfoy, Walden Macnair, and Magnus Warwith were all gathered around a large tree with a hole dug under its roots, and Lucius was carrying a large, heavy-looking bag. Standing absolutely still behind a group of bushes - which helped support the Notice-Me-Not spell - Snape watched as Macnair held out an old shoe. The other two touched it, and all three vanished as the Portkey activated.

Moving away from the bushes, Snape walked over to the tree and examined it. He wasn't terribly surprised to find that the 'hole' was in fact a protected hollow, and he could sense the fading remnants of a decades-old concealment spell. This was undoubtedly the location of one of the other caches the Death Eaters had still been in the process of finding as of yesterday morning.

Only one or two caches left, if that, which they need to find, Snape reflected, irritation and anxiety mingling and bringing his headache back almost full-force. He needed to get word to Moody, Weasley and Dumbledore at once.

A wave of his hand placed a marker on the spot, to make it easy for him to find again, and he departed at a near-run toward the school.

A hastily-written note was dispatched to Dumbledore via the message-box, giving the headmaster all the details of what he'd observed and discovered, and then he Flooed Arthur Weasley.

By some incredible stretch of luck, Moody was also in Weasley's office. As the Floo connected, Snape heard him saying, "...four caches in London, and one more in Edinburgh--"

The ex-Auror broke off and turned around to look directly at him. "Snape."

"Moody," Snape returned curtly. They disliked each other, but they had developed a certain wary respect over the past sixteen years, based on the work each of them did for Dumbledore. "I have some urgent news concerning the caches."

"What is it?" Weasley demanded.

"There was a cache just outside the wards of Hogwarts," Snape replied, and quickly filled them in on what he had seen.

"Not good," Moody muttered darkly.

"That leaves them just one or two more," Weasley added, echoing Snape's own thoughts. "And we've already found eight others. Do you have any suggestions on how to help narrow it down?"

Snape frowned, but recalling his research from this morning, replied, "Tell Fletcher and the mutt," his mouth twisted in distaste, "to check if any of them contain Fire Crab shells. Ground to a powder, it helps to stabilize Dementor's blood in a broad range of Dark Potions."

"All right; thank you, Professor," Weasley said, while Moody regarded him narrowly.

He felt no need to defend himself to the ex-Auror. "One more thing," Snape continued, keeping his eyes on Weasley. "The reason I was in the Owlery in the first place was to send you a letter, but since I'm speaking to you, I may as well ask you now.

"The wards on the secured facility. You mentioned in your original letter that you would be alerted if they were broken. How? Were they set by one of your people, or were they alarmed via the Ministry?"

Weasley frowned thoughtfully. "I believe one of my people set the alarms up to come directly here, but I'll check," he replied.

"Send your answer back by the owl," Snape told him, and pulled away from the fire, ending the conversation.

Glancing up at the staff wizarding clocks on the wall of Dumbledore's office, Snape nearly groaned when he saw his was pointing to Time to Tutor Potter. Firmly reminding himself that it had been his idea in the first place, and he had volunteered for the role, he stood up, brushed off his robes, and stalked out the door.

Harry frowned as he waited outside Snape's office. The door was closed; he'd knocked, but there had been no answer. He couldn't help but wonder if something was wrong; when Dobby had shown up at noon to tell him that the lesson had been changed from one to two, the house-elf had also said that apparently 'Professor Snape sir' wasn't feeling well.

Despite the fact that he still disliked Snape (no matter that he was coming to respect the greasy git), Harry was worried. If Snape wasn't well, he didn't know how to help--

"Potter," came Snape's voice suddenly from behind him, and Harry spun around with a startled squeak.

The Potions master eyed him in irritation, heavily-flavoured with dislike. "You are going to have to do much better than that, Potter, if you want to survive another year," he snapped. "You should have known I was there before I spoke, and been ready to defend yourself if necessary. Death Eaters won't warn you they're behind you, any more than Voldemort will. You must learn to pay attention to your surroundings."

Harry blinked at this completely unexpected tirade. Snape's tone of voice sounded the way it always had during the school year, all of a sudden; but rather than insults and accusations, Snape was talking about the lessons and his safety. It was... bizarre, and left Harry feeling so confused that all he could do was stare at Snape in utter bewilderment.

Of course, Snape's response to that was entirely predictable. "Have you lost what little wits you have, Potter?" he sneered.

"Sorry, sir," Harry said quietly, looking away. He could feel Snape's glare burning into his head, but didn't respond. Has something happened? Why is he being so... so much of a git all of a sudden? But he still seems concerned...

"Just get in, Potter," Snape ordered curtly, opening his office door.

Harry obediently walked in and sat down in what had become his regular chair over the past week, then pulled his work out of his bag and waited for Snape to demand it. And while he waited, he took the opportunity to study the Potions master carefully.

Surprisingly, Snape looked tired. Harry had never seen that before, even on days when he knew Snape had been up and around the entire night before. His eyes, which were usually glittering with emotion - generally malice, or, in the past week, irritation, when directed at Harry - looked uncommonly dull.

Snape finished shuffling the papers on his desk, then held out his hand and gave Harry a pointed look.

"Here's my assignment, sir," Harry said carefully, giving him the scroll with his list of environmental characteristics, "and here's the description of the chill from the visions."

Snape took both with a nod, and placed them on his desk. Then he picked up one of the piles of paper, revealing a book under it, and thrust the papers at Harry.

"This is a copy of the introduction to this book," Snape said, gesturing toward the book on his desk. Harry noticed that the viciousness which had been in his voice had now disappeared again. "The book is entitled Sensitivity: Learning the Basics, and is an excellent manual for beginning to learn how to use the Sensitive's gift. Today's assignment will be to read through that material and ensure that you know it thoroughly before your next lesson. This is one subject for which I will definitely refuse to tolerate any skiving off."

"Yes, sir," Harry replied simply. He didn't want to irritate Snape again - he much preferred the Snape he'd been dealing with this past week to the one he'd known for the last four years.

Snape sat back down behind his desk, steepled his fingers, and met Harry's gaze. "What do you think of How to Defend Yourself Without a Wand?"

Harry blinked, startled again, although this time it was due to being asked for his opinion - very definitely a first from Snape. He hesitated - then, as Snape started to look impatient, hastily blurted out, "Professor Dumbledore thinks it's well-written."

Snape didn't appear to be in the slightest bit impressed. "That is the headmaster's opinion," he said, his tone giving no clue as to whether or not he agreed with it. "I asked for yours. And do not try to tell me you don't have one - we are both aware that you have opinions on everything, Potter, whether you voice them or not. What do you think of the book?"

"That it was written by a Slytherin," Harry replied.

Snape regarded him calmly. "Why?"

"Because there's so much emphasis on the fact that wandless magic gives you secrecy and surprise."

"Is that all you think about it?" Snape questioned.

Should I? Harry wondered for a moment. I guess so; he has been telling me that this isn't like class... "I was a bit surprised there wasn't more about physical methods of defence," he answered. "I was sort of expecting that to be the main topic, and instead it was only one section."

"It was written by a wizard for witches and wizards, not Muggles," Snape pointed out.

Harry paused, then shrugged mentally. Might as well... After all, it's not like I've never argued with him before. "Just because we're not Muggles doesn't mean that there's no reason to use physical means of defence," he countered. "After..." He hesitated, then steeled himself and continued, "After the Third Task, I was able to avoid some of Voldemort's curses by running and ducking. If I hadn't, I'd be dead."

Snape studied him for a long moment, then nodded. "Good," he declared, surprising Harry. "At least I will not have to waste time attempting to teach you that running away can be a better decision than staying to die because of idiotic Gryffindor pride."

Harry winced slightly at that, but held his tongue, consciously choosing not to argue with Snape's statement; he'd returned to the way he'd been acting for the past week, and Harry really didn't want him to go back to normal. Especially when he'd just received a drastic demonstration of how different Snape was now.

Snape appeared to be pleased by his reaction - or, at least, by his lack of argument. Well, that's no surprise, considering how much he hates Gryffindors...

"You will no doubt be pleased to know that I agree with your opinion on the lack of importance of physical defence in the book; yet it is anything but unique in that respect. Most books on wandless defence pay little or no attention to the physical aspect. When dealing with wandless magic, however, How to Defend Yourself Without a Wand is by far the most useful book available, which is why I chose it.

"For today, we will concentrate on the theoretical aspects of wandless magic. Next week, you will start a physical training regimen, but that will wait until you have completely recovered from the damage due to your visions," Snape finished.

"Yes, sir," Harry said, automatically pulling out parchment, his quill, and ink.

"First of all, there are very definite limitations on wandless magic, even for Sensitives; our limitations are different from those of the general wizarding population, but they do still exist," Snape said, as Harry quickly began to scribble down notes. "We will not be dealing with those today; you will need to read through that introduction before we discuss any further details of the Sensitive's gift.

"While the general limitations of wandless magic do not apply to either of us, they do apply to everyone else you know, both allies and enemies; therefore, you need to understand how it works."

"How to Defend Yourself said that wandless magic is magic taken from the wizard's own stores of magic, whereas using a wand uses magic from the environment," Harry said.

"And do you understand what that means?" Snape questioned.

Remembering Snape's reaction on Monday to his hesitation over asking a question, Harry immediately shook his head. "No, I'm afraid not, sir," he replied.

Snape nodded as though he had been expecting that response - which he probably had been. "Yes, that is the one area - aside from physical defence - where How to Defend Yourself is rather poor," he commented.

"To put it simply, all magical creatures - including witches and wizards - are born with a core, or source, of magical energy. That source can only contain a finite amount of magic - the amount is different for each person, and together with the speed with which the source can replenish itself determines how powerful you are. If you have a great deal of magic, but it replenishes itself slowly, you tend to be more careful of using it than someone in the reverse situation."

"Replenishes?" Harry repeated, zeroing in on the term whose precise meaning in this context he wasn't sure he understood.

Snape nodded. "There is magic all around us - in the air, the earth, the water; once you are trained in using your gift, you will be able to sense it. That magic - and there are hundreds of different theories concerning its origin and why it appears to be infinite - is what replenishes both our own sources of magic and those of other magical creatures. It is known as 'ambient magic'.

"Doing magic using our own source, and only our own source, is wandless magic," Snape continued, "and until wands were invented, that was all wizards could do."

Harry nodded in understanding as he finished writing. "So... wands allow us to use the ambient magic as opposed to relying only on our own," he concluded.

"Yes, in essence," Snape replied. "A wand is made up of two elements: something from a magical, non-human creature, which is the core of the wand; and a type of wood, generally one with strong magical properties. The core, coming as it does from a magical creature, can access the ambient magic; the wood serves to help contain and direct that magic, so it can be controlled.

"Even with a wand, however, you always use some of your own magic; part of it to start the draw on the ambient magic, and the rest to help shape the magic into what you want. That is why, first of all, getting the correct type of wand is so important: if a wand isn't compatible with your own source of magic, it will produce either a very weak result, or something that is generally the opposite of what you intended - and often rather nasty.

"Secondly, it means that the more powerful the spell you cast, the more power it requires from you to direct the ambient magic."

Harry frowned slightly. "When we covered the Unforgivables, Crouch said that we could all cast the Killing Curse on him, and it wouldn't give him any more than a nosebleed." He didn't like talking about this particular curse - or any of the Unforgivables, for that matter - but he wanted to understand. "Is that why?"

Snape nodded. "At the moment, the full force of the Killing Curse would require more of your own power than you have available. Or that's true for your classmates, at least. I'm not certain about you personally. Of course, it also - as for all the Unforgivables - requires that you have the genuine desire to cast it."

Harry hadn't really heard that last sentence; he was too busy gaping at Snape in shock. Snape, admitting he didn't know something about Harry? It was... well, inconceivable! Snape had always claimed to know exactly who and what Harry was. He'd been wrong, of course, but still...

"Potter! Pay attention!" the Potions master snapped sharply.

"Sorry, sir," Harry apologized, still stunned, but knowing much better than to make an issue of it. Instead, he returned his focus to their conversation about power. "But, if spells like the Killing Curse would require more power--"

"I said 'at the moment', Potter," Snape interrupted.

Had Snape been taking mind-reading lessons from Dumbledore? Harry was starting to suspect the answer was 'yes'; this was the second time Snape had responded to what he was thinking, as opposed to what he had finished saying out loud, after all.

"As children, untrained, it takes a great deal to access the power for more than the simplest of spells - and they don't have access to the entirety of their own magic source. As a witch or wizard matures physically, they gain greater access to their power; and as they are trained, and practice, they develop a more delicate touch, learning to do more with less power. You - and your classmates - could now do first year spells with much greater ease and without the large expenditure of power it took when you were first learning them. That is why.

"Despite the fact that it takes less from you the more training and practice you have, a spell will always need the same amount of power to produce the same result - which leads back into the actual topic we are discussing. While more complicated and powerful spells can be done with wands as you learn more, the same cannot be said for wandless magic. You may need a bit less for focusing the spell, but - to get consistent results - the spell itself will always take the same amount of magic whether you are a first year or a fully trained wizard."

"Which means that a wizard can't do a wandless magic spell that requires more power than his source contains," Harry concluded.

"Precisely. If they try, they either simply cannot do it, or they burn themselves up, becoming squibs. So," Snape continued, "do you understand now what the book was trying to say?"

"Yes, sir," Harry replied. He paused for a moment, and then asked, "Is there any way to tell how powerful a wizard is?"

"Not with complete reliability," Snape replied, leaning back in his chair and steepling his hands under his chin. "Not even for Sensitives. An estimate can be made, based on a student's progress throughout their schooling, and the power they are able to put into various spells - for example, the Patronus you summoned at that Quidditch match your third year. Despite the fact that it was unnecessary - as there were not truly any Dementors there," Snape's face twisted into an expression Harry couldn't read, although he thought there may have been disgust involved, "the fact that you, as a mere third year, were able to conjure a fully-formed Patronus was telling.

"Of course," he added, his tone somewhat acidic, "it would be even more impressive if you showed any other signs of such power... or any sign of wisdom in using it."

Harry winced slightly, wondering what was going on with Snape as he did so. The Potions master's mood seemed to fluctuating wildly, and Harry wasn't entirely certain how to react. Especially when Snape continued as though he hadn't just been sniping at him.

"Other than that, the only way we have to measure a witch or wizard's power is by comparison with other witches and wizards. For example, we know that Dumbledore is more powerful than Voldemort - but not by how much," Snape explained.

"However, we have once again strayed from the point of this lesson.

"All wizards have some degree of wandless magic - they would be unable to even use wands if they did not - but more than half the wizarding population has no more than what is required to use a wand and do the most basic of uncontrolled spells.

"Which leads into the next limitation of wandless magic; it is very, very difficult to control."

Harry, remembering the times when he'd inadvertently used wandless magic, nodded in agreement; and then felt his stomach sink as Snape smiled tightly. He probably shouldn't have done that... It was, to quote something Uncle Vernon had once said, like waving a red flag before a bull.

"And why, Potter, is it so difficult to control?"

It wasn't all that hard to figure out, actually - not when he'd just refreshed his memory of the incidents. "Because wandless magic responds to emotions, rather than to focused thoughts," Harry replied.

"Not entirely correct," Snape said, shaking his head. "Yes, wandless magic responds more readily if strong emotions are involved, but that is not what it truly responds to."

Harry frowned slightly, trying to figure it out. It hadn't been in the book, and if it wasn't emotions... Finally he shook his head in defeat. "Sorry, sir; in that case, I really don't know."

Snape looked irritated again. "Your subconscious mind, Potter," he growled. "That is why quite often the first manipulation of magic in a witch or wizard is either in reaction to some form of danger, or a temper tantrum.

"Because of that, in order to truly control wandless magic, a witch or wizard must have control over their subconscious mind, as well as over their emotions. Very few people can achieve such control."

Harry nodded again, and Snape was just about to continue when the door opened and Dumbledore walked in.

"Professor Dumbledore?" Harry had thought Dumbledore was going to be away until Monday or Tuesday...

"Headmaster?" Snape, he realized, sounded equally surprised.

"I am sorry for interrupting your lesson, Severus, Harry," the headmaster apologized. "However, I'm afraid I must ask you to cut it short. Severus, I just received your latest report; I need you to show me where it was." Dumbledore looked quite agitated, Harry suddenly noticed, and he wondered what Snape's 'latest report' had been about.

"Of course, Headmaster." Snape stood up, then turned to Harry. "Your next lesson will be in the Potions classroom on Monday at one, Potter. In addition to the reading assignment I gave you at the start of this session, I would like you to re-read chapter fourteen of How to Defend Yourself Without a Wand, and learn the list of limitations there, taking into account what we have discussed this afternoon. Be prepared to discuss them in more detail Monday."

"Yes, sir," Harry replied obediently, even as the two professors hurried out of the room. Confused, and more than a little worried as he thought about what might have prompted such a reaction from Dumbledore, he slowly packed up his things and walked up to Gryffindor Tower.

"What's the matter, Albus?" Snape asked, once they'd got far enough away from his office that Potter wouldn't be able to eavesdrop. "Why the sudden urgency?"

"According to your note, this cache you saw Lucius empty was just outside the Hogwarts Wards," Dumbledore said, his tone grim.

"Yes," Snape agreed slowly.

"That should not have been possible," Dumbledore continued, as they reached the Great Hall. "Even outside the wards, even with a concealment spell, I should have known about it if it was that close. The fact that I didn't concerns me a great deal, and makes one wonder why I didn't. And I don't like the possibilities I have come up with. In order to know for certain, however, I must actually see the location of the cache myself."

"Of course," Snape replied. He had no idea what Dumbledore was expecting, but it was obvious that the headmaster was definitely worried about the situation.

Leading the way out the main doors of the school, Snape started in the direction of the tree that had hidden the cache, activating the beacon he'd set as he did so.

Ten minutes later, they reached the tree. As soon as Snape pointed out the hollow under its roots, Dumbledore crouched down and began examining it carefully. He spent almost half an hour on it, while Snape kept an eye on their surroundings.

Finally, Dumbledore stood up and beckoned to him. "Severus, could you please do me the favour of examining this?" he requested, gesturing to the hollow. As a Sensitive, he meant.

Snape frowned. "What am I looking for?" he asked, even as he joined the headmaster and began to reach out to scan the interior.

"Anything that may explain why no one sensed this cache," Dumbledore replied. "There had to be something more than just the concealment spell."

Such a vague idea of what he was looking for made it rather difficult for Snape; usually, he had more information about - and a much more concrete idea of - what he was trying to sense. Without that, it could be compared to walking into a room in complete darkness, and trying to find a red cushion - he might be able to find a cushion, or several, but he would have a difficult time determining the colour.

He sensed the still-fading concealment spell first and pushed past it, stretching his Sensitivity out in an effort to feel whether or not there was anything else.

That... was a very big mistake, Snape decided two minutes later, when he'd finally stopped retching.

"What happened, Severus?" Dumbledore asked anxiously.

"Too much taint," Snape managed to mumble, staggering as he attempted to stand back up.

"Taint?" Dumbledore repeated, gripping his arm to steady him. "I assume you mean twisted magic. Can you clarify it at all, Severus? Was it a particular type of taint, or simply general Dark Arts?"

"I've told you before, Albus - 'Dark Arts' aren't twisted; it's the way they're used," Snape returned, irritated - a feeling which faded to exasperation when he noticed Dumbledore's eyes were twinkling. Albus is much too good at that, he thought, as he leaned against a tree. This wasn't the first time the headmaster had provoked him in order to let his anger provide him with an extra bit of strength.

"There was definitely something there that involved twisted magic," he continued. "It felt... almost like a smothering blanket - literally." That wasn't an exact description for the twisted, tainted thing whose residue he'd sensed - there were no words to convey it properly to a non-Sensitive - but it was close enough to give Dumbledore at least the general idea.

"A smothering blanket," Dumbledore repeated thoughtfully.

Snape's mouth twisted in remembrance. "That... wasn't the worst of it, however," he said, his voice hushed.

Dumbledore met his eyes. "What was?"

Snape took a deep breath, let it out slowly, then took another one - trying to settle his nerves. Bad enough he'd spent the entire morning dealing with unshielded, tainted books in his office; but not even the worst of those books compared to what he'd sensed from the hollow. "Whatever that thing was that left the residue I sensed..." He hesitated for a moment. "It was done by... Only a Sensitive could have created it."

That was the worst of it. Knowing how twisted magic affected him, Snape had no idea how any Sensitive could deliberately create something that twisted magic in the way whatever that was had.

The twinkle had almost vanished from the headmaster's eyes, and he looked more than a bit alarmed. "Oh, dear..."

Although he wasn't quite certain of the exact reasons for the alarm, Dumbledore's concern increased Snape's own. If the headmaster was worried, the rest of them should probably be terrified. "What is it, Albus?" he demanded.

"Have I ever told you about Portia?" Dumbledore asked in response, starting back toward Hogwarts.

Snape frowned thoughtfully. The name sounded vaguely familiar... "That classmate of yours who turned out to be a Sensitive," he said a moment later, as he remembered Dumbledore telling him about Portia Bones after the headmaster had realized what was happening to him.

"Yes." Dumbledore's expression turned grim. "During the war against Grindelwald, she had a rather unfortunate encounter with something that - based on what I remember of her description - sounds like what you've just described."

"Oh?" Snape prompted him.

"Someone on Grindelwald's side - possibly even Grindelwald himself - was a Sensitive," Dumbledore explained. Snape's eyes widened in surprise. "A Dark Sensitive - damaged somehow, I believe," the headmaster continued. "He discovered some rather unusual traits of Dementor skin."

Despite the fact that he routinely collected potion ingredients the students - and even most adult witches and wizards - would consider disgusting, without a trace even of distaste, Snape felt himself grimace in absolute and utter loathing at the very thought of Dementor skin.

"It seems that if you put an Obscurus Charm on the skin, and bind it - in a way that only a Sensitive can accomplish, according to what Portia discovered - it not only hides the skin itself, but anything it is wrapped around, from the magic of others. Which means that whoever owned this cache had to have had access to things from Grindelwald.

"And now, Voldemort has access to this Charmed Dementor skin, and Dementor's blood." Dumbledore stopped walking for a moment, and when Snape looked at him, met his gaze firmly. "I have no desire to find out what he could do with that combination, but we must know. I fear I must ask you to do more research for me."

Snape nodded in agreement as they began walking again. "Will you be returning to Hagrid?" he asked a moment later, as they reached the main doors.

"No," Dumbledore replied. "I've done what I needed to there, and with the way things seem to be going, I think it's best I return now.

"So," the headmaster continued, "now that I'm here in person, why don't you tell me what else is bothering you about young Harry - what it was you didn't want to mention over the Floo connection."

Snape sighed. "These visions of his - I'm concerned about what they're doing to him - and not just in the sense of the effects of Cruciatus," the Potions master said grimly. "Judging from the report he wrote for you of his vision Wednesday night, you know about the torturing and killings he's been seeing."

Dumbledore nodded, his own expression sombre. "Yes, I know."

"We have to stop them," Snape said firmly. "The information they can provide may be valuable, but the risk to Potter's sanity is unacceptable."

"He's not fragile, Severus," Dumbledore said quietly. "In fact, young Harry is remarkably resilient."

Snape stopped walking abruptly and stared at Dumbledore. The headmaster could be very precise in his choice of words when he wanted to be, and something told Snape that this was one of those times. "'Resilient'?" he repeated. "I would have said 'strong'." And the strong can be broken - much more easily than the resilient. "That is, after all, a Gryffindor trait."

"No," Dumbledore replied calmly. "Oh, there can be no doubt Harry is strong, that's true enough, but it is his resilience that keeps him from being fragile. As for House traits..." The headmaster smiled, and Snape felt a sinking feeling in the pit of his stomach. "Gryffindor was not the Sorting Hat's first choice of House for Harry Potter."

The sinking feeling grew stronger - but he had to know. If the answer was what he suspected, changes would have to be made. "Which House was the Hat's first choice?" Snape asked.

Dumbledore's smile widened. "Why, Slytherin, of course, Severus," he replied, his eyes twinkling brightly again, and started walking away.

Snape stared after him for a moment - and then abruptly realized that Dumbledore had deflected his point about stopping Potter's visions very neatly. Pushing the latest revelation to one side, to think about later, he called, "Albus!" as he stalked after Dumbledore.

"Yes, Severus? What is it?"

"We have to find a way to stop Potter's visions," Snape repeated. "Resilient or not, he should not have to see--"

Dumbledore held up one hand, and Snape stopped. "Severus..." The headmaster sighed. "I don't think they can be stopped - at least, not until Voldemort is completely dead."

Snape frowned. "Surely there must be some way to block the link from his scar," he protested.

"Perhaps," Dumbledore admitted, "though I have been unable to find one in the past four years, for all my searching. However, I don't believe the link through Harry's scar is the cause of most of these visions - at least, not directly."

Snape blinked in surprise. "You don't? I would have thought that would be the obvious answer."

"The obvious, yes, but... Before Voldemort's resurrection, Harry had only two visions, Severus. One last summer, and one about a week after Bartemius Crouch's death, shortly before the Third Task.

"I told you what Harry told me about the events in the graveyard the night of the Third Task," Dumbledore continued, and Snape's eyes widened in surprise - and shock - as he realized what the headmaster was getting at.

"His blood," the Potions master breathed. "You think it's his Sensitive's gift in combination with the link at his scar and Voldemort's use of his blood." Magic was bound up in the blood - even the greenest first year student knew that. And a Sensitive's blood...

Dumbledore nodded. "Harry said himself that the visions changed after the Third Task."

This was not good. If Dumbledore was right about the direct cause of the visions - and there was a very good chance he was, now that Snape knew what his theory was - then he was also right that only Voldemort's final death would break that binding. Although, perhaps as he trained Potter in the use of his gift, they might be able to find a way to block it...

However, that wasn't the only thing troubling him about Potter's visions.

"That still doesn't explain the Cruciatus effects," he pointed out.

"No, it doesn't," Dumbledore agreed. "Now, if you will excuse me, Severus, I had best contact Arthur and tell him I'm back - and let him know about the Dementor skin." With that, he headed down the corridor in the direction of his office.

Snape watched him go, then sighed and turned back toward the dungeons. Dumbledore obviously had no intentions of giving him even a hint as to what his theory about the Cruciatus effects was.

There being nothing he could do about either Potter's visions or the problem of the Dementor skin just at the moment, his thoughts went back to the most startling revelation of the past hour - the truth behind Potter's Sorting.

If Slytherin was the Hat's first choice for Potter, I wonder how he persuaded it to put him in Gryffindor, Snape mused, as he descended the stairs to the dungeons. He must have had about the same amount of traits and enough of the mindset from each House, since the Hat relented and allowed him to choose. Not that Dumbledore had specifically said that it had been Potter's choice, but it was obvious.

Of course, having been in Gryffindor for four years meant that Potter's Gryffindor mindset and traits - courage, strength, recklessness and pride, to name the most obvious - had been encouraged at the expense of others; but if he'd had enough Slytherin in him for that to be the Hat's first choice... he would still have traces of that. Traces that Snape should be able to hone and strengthen - at least enough to help ensure the boy's survival.

He would have to revise his lesson plans, of course - they'd been based on his observations that the boy was a Gryffindor to the core. The revelation that Potter was at least part-Slytherin meant that there would be other - better - ways to teach him than through a completely Gryffindor model.

It would be an interesting challenge - one Snape suspected the headmaster knew would intrigue him enough to keep him interested.

He had originally been uncertain about leaving Severus and Harry alone for several days; but judging from the lesson he'd interrupted, they had at least decided on a truce - which might just turn into the first step toward a true mentorship bond. His smile widened as he remembered the look on Severus's face when he'd mentioned the Sorting Hat's original decision; Dumbledore suspected that that knowledge might just prove to be the catalyst for another step or two along that route for the two of them.

Better by far that they established at least a good start on a mentoring relationship before they found out why Harry was experiencing the vision curses. A well-established mentor/student bond would be best, but Dumbledore had the feeling that one or the other of them would discover the truth before they got that far.

Then he sighed. As much as he wished to contemplate the pleasant thoughts of Severus and Harry finally learning to work together without the viciousness and anger that had characterized their relationship since Harry's first year, there were other concerns he had to deal with. Going over to the fireplace, he picked up his container of Floo powder and prepared to contact Arthur Weasley with the latest news.