Disclaimer: JK Rowling owns all recognizable elements from the Harry Potter universe. The offset and italicized section is a direct quote from hardpack US edition of PoA, page 340.

Price of Magic, Depths of Soul

"Indubitably, Magic is one of the subtlest and most difficult of the sciences and arts. There is more opportunity for errors of comprehension, judgment and practice than in any other branch of physics."
-Aleister Crowley

"The incantation again, Harry –" Lupin cleared his throat. "Expecto Patronum."

Harry sighed briefly before shrugging his shoulders back, preparing himself to once more cast against the boggart Lupin had provided. His last lesson had, despite his best efforts, not been quite as successful as he had hoped. It wasn't that the charm seemed too terribly difficult; Harry simply couldn't find a memory strong enough.

Nothing, it seemed, lasted against even a faux-dementor, and after every attempt at it his range of potential options grew slimmer.

"Are you ready, Harry?" Lupin prodded gently, resting his hand upon the iron curve of the trunk's latch. Harry nodded shortly, shifting his stance slightly in preparation to cast.

"Ready."

Lupin's left hand deftly flipped open the latch, releasing shadow and smoke out into the room, coalescing briefly into a luminescent orb before shifting into the familiar black cloak of a dementor. As Harry's senses began the slow blur into a grey mesh of oblivious fear, he raised his wand, noting – and taking some irrational measure of comfort from it – that the professor had his wand firmly in hand, obviously prepared for any eventuality.

"Expecto Patronum!" Harry shouted while concentrating on the moment he had first laid eyes upon Hogwarts, but with the knowledge of Lupin's vigilant presence niggling in the background as well.

As before, a silvery mist shot from Harry's wand to hover in front of him, but this time it was different. An animal of sorts – four legs definitely, but poorly defined – emerged as shades of darker grey the colour of steel twisted through the being. The boggart made no retreat, but halted its previously steady advance, allowing Harry a long moment of respite from the haze of fear and sorrow he had slipped into.

Before the patronus began even to fade, Lupin stepped quickly in front of the boggart, and watched calmly as it returned to an orb. As the professor prodded the now relatively docile boggart back into its trunk, Harry noticed that he seemed, perhaps, a little more weary than before.

Harry shrugged off the perception; anyone would be a little worn down by the presence of a dementor, even if it was preternaturally attracted to someone else in the room.

The thought fled from Harry's mind, however, as Lupin began to speak. "Excellent job, Harry!" the professor enthused, momentarily becoming more animate than his usual, somewhat distant tone. "You almost had it there; I'm certain that with one or two more lessons you'll be able to cast a corporeal patronus."

Harry's mouth stretched into a wide smile. "That was amazing; I couldn't even feel it at the end! Do you think it'll work that well on a real dementor, professor?"

Lupin's own smile thinned slightly, the pride in his pupil's accomplishment receding, showing as he returned to his typical detached and professional tone. "It should in theory," said the professor, "but you must keep in mind that dementors are rarely encountered alone, and never in such controlled circumstances as this."

His smile dimming somewhat, Harry's voice took on a subtle air of determination, a hint of the iron will that shone in truly dire situations trickling through. "What can I do to make sure it works then?"

"I don't mean to belittle your accomplishment, Harry," replied Lupin, "what you have done already is unheard of for a third year, and there are many adults who have never mastered this charm. I believe that you can do this, Harry, that you can cast a corporeal patronus with a little more work. You need to concentrate on a single memory, though, a single moment of absolute happiness."

Harry thought for a second, recalling everything he had done to cast the charm. "Is that why it wasn't fully formed, sir, and why it wasn't quite silver everywhere?"

Lupin nodded at the first question, but his brow tightened slightly when Harry mentioned the colour. "You concentration certainly affects the definition of your patronus's form, but I frankly do not know what would affect its colour." His brow wrinkled further. "You are quite sure it was not silver throughout? I'm afraid I didn't see another colour."

Harry looked a little sheepish as he replied, "Well, sir, I thought some of it looked grey... instead of silver, you know..." He blushed slightly, "It was probably just the light or something; it wasn't a terribly different colour."

A soft chuckle escaped the professor as he wryly spoke, "Yes, quite." He glanced at the clock before continuing, "It's getting rather late; perhaps it would be good to end here for today." As Harry began to protest, Lupin raised a hand briefly, forestalling him. "Yes, I know there is still time before curfew, but I feel rather more tired than I ought, all things considered. The fatigue from my recent illness isn't entirely gone, I suppose."

A brief scowl appeared on Harry's face, both at the prospect of a shortened lesson and the unfortunate reminder of Snape's time in the Defense classroom. It vanished just as quickly, however, as Harry remembered how tired the professor had looked putting away the boggart. Not wishing to further disturb Lupin, he made his way to the door.

"Alright Professor," he grinned, "as long as it isn't Professor Snape in Defense tomorrow I don't mind cutting it a little short.

Lupin smiled briefly in return, assuring Harry that he was just a little fatigued as he shut the office door behind his student.

As the lessons continued, Harry was finally able to produce a corporeal patronus, though it was still only under Lupin's watchful eye. There were still, to Harry's eye, streaks of cold grey within the silver, but they lessened with each attempt.

It was several weeks later, during the match between Gryffindor and Ravenclaw, that Harry found an opportunity to cast a patronus outside the confines of Lupin's office.

Ravenclaw was catching up to an early lead by Gryffindor; they had scored three goals, which put Gryffindor only fifty points ahead. After being blocked out by Ravenclaw's seeker, Cho Chang, the first time he spotted the snitch, he was ecstatic to spot it again hovering near the Ravenclaw goalposts. Harry accelerated rapidly, glimpsing Cho doing the same just beside and below him. Harry knew he was winning, gaining on the snitch with every second, then –

"Oh!" screamed Cho, pointing towards the sidelines.

Distracted, Harry looked down. Three dementors – three tall, black hooded creatures – were looking up at him.

He didn't stop to think. Plunging a hand down the neck of his robes, he whipped out his wand and roared, "Expecto Patronum!"

He concentrated on another happy memory as he cast, but kept the corner of his eye on the snitch hovering just ahead. He knew his patronus was fully formed – he couldn't feel the dementors at all – and so shifted his mind fully to the task at hand, grasping the snitch in the tips of his fingers.

Harry landed on a field in chaos; the Gryffindors in the crowd were celebrating madly, having clenched the Quidditch cup for a third straight year. However, rather than join the celebrations, Harry paused as a voice behind him said, "That was quite some patronus."

Harry turned around to see Professor Lupin, who looked both shaken and pleased.

"The dementors didn't affect me at all!" Harry said excitedly. "I didn't feel a thing."

"That would be because they – er – weren't dementors," said Professor Lupin. "Come and see –"

He led Harry out of the crowd until they were able to see the edge of the field. "You gave Mr. Malfoy quite a fright," said Lupin.

Harry stared. Lying in a crumpled heap on the ground were Malfoy, Crabbe, Goyle, and Marcus Flint, the Slytherin team Captain, all struggling to remove themselves from long, black hooded robes. Both Professors McGonagall and Dumbledore were standing over them with obvious displeasure, but, upon catching sight of Harry, Dumbledore extricated himself and walked towards him.

"Ah, Harry, that was a rather... interesting way to end a match," intoned the Headmaster. "If you would consent to missing the early celebrations here, perhaps I could speak with you about it in my office?" Dumbledore smiled. "Doubtless it will give Messrs. Weasley ample time to prepare a fitting celebration in your common room."

"Of course, professor," replied Harry, waving goodbye to Lupin and acquiescing to be lead across the grounds towards the entrance. As the pair passed through the main doors and began the trek up to Dumbledore's office, the Headmaster started to speak.

"It is quite an accomplishment to be able to cast a patronus at your age, Harry. May I assume Professor Lupin has been teaching you?"

"Yes, sir," Harry responded. "He's been giving me lessons since the beginning of term."

Dumbledore's eyebrows raised slightly. "Sherbet Lemon," he crisply spoke to the gargoyle at the entrance to his office. "It is an even greater accomplishment to have learned this so quickly."

Harry's thanks were forestalled as Dumbledore sat down in his chair, gesturing for Harry to take a seat as well. "However, there are reasons that the patronus charm is not typically taught to third years; indeed there are reasons few wizards learn it at all.

"Sir?" Harry spoke, the question carrying easily in his voice.

"Did you perhaps glimpse your patronus today, Harry?"

"No, sir." Harry looked puzzled at the apparent non sequitur. "I've seen it before though; I've cast a corporeal one with Professor Lupin. It's a silver stag." Harry paused slightly before continuing, "There might be a bit of grey in it too, sir, but I'm not really sure."

The Headmaster looked at Harry and spoke, "Your patronus today, Harry, was indeed a silver stag and was, I assure you, quite well formed. However, it was streaked with gold – not a great amount, but it was quite unmistakable." Dumbledore sighed briefly, noting Harry's surprise, before continuing, "I know that this is a very personal question, but I must ask: what were you concentrating upon when you cast your patronus today?"

Harry shrugged, "I don't mind answering, sir. It was pretty much the same thing as always; I used a happy memory of Hermione, Ron, and I. I don't remember exactly what." Harry paused a moment, missing – in his pensive state – the briefly stricken look that flashed across the Headmaster's face. After a beat he continued, "I suppose I was also concentrating on the snitch still." He looked up again, appearing somewhat disappointed. "It wouldnt've worked with a real dementor then, would it? I wasn't concentrating enough."

Dumbledore sighed again before speaking in a surprisingly earnest tone, "Harry, the patronus charm is powerful magic, and poorly understood. Most wizards lack the focus to concentrate upon a truly singular instant without first ordering their minds, harnessing and protecting their very Will with magic. There is an entire branch of magic – Occlumency – dedicated to this, Harry, and it takes years to master."

Harry opened his mouth to ask another question, but Dumbledore interrupted quickly, "Please, Harry, let me explain everything before you ask any further questions." The Headmaster began speaking again as Harry gave a short, somewhat impatient nod of affirmation.

"The impurity of your patronus, your inability to remember exactly what memory you used – these are dire signs, Harry. That your patronus was flecked with gold is, given what you recall, doubtless a simple reflection of your concentration on the snitch. It is a fairly inconsequential mistake, as harmless as one may hope when dealing with truly powerful spells." Dumbledore paused again. "Tell me Harry, what have you learned about magic itself?"

Harry frowned in thought. "Erm, is it about willpower, sir? That you have to want something, command it to change?"

"Indeed, Harry," the Headmaster replied, smiling only briefly at the answer. "Willpower affects change, as is its nature. But powerful magic, spells of creation and destruction – it requires sacrifice." He glanced at Harry's scar. "You are familiar with one form of this, but life and death are not the only applications. Dementors, you would agree, are creatures of destruction, destroying hope and happiness at best, and entire souls if given the chance."

A brief shiver went through Harry at the reminder of the dementors' most terrible weapon, but he nodded at the Headmaster to continue.

"The patronus charm counters this destructive force, creating a corporeal being of absolute hope and happiness distilled from a single recalled instant of the feelings. That instant," Dumbledore leaned forward, imploring Harry to understand the gravity of his words, "is sacrificed."

Harry paled rapidly as the realization hit. He had never used the same memory twice for a patronus, but he had never thought about it before. "But... how... how am I supposed to keep casting patronuses?" Harry blanched. "Can I run out of memories?"

"Properly cast, you cannot," said Dumbledore. "It is why occlumency is learned first, so that a wizard may take a true instant in time, and not the complete memory associated with it. Casting as you have been, Harry, you will eventually run out of suitable memories. The memories may be the least of things though, Harry. Magic of this nature is rarely studied and little known, but losses of this form...," the Headmaster looked suddenly far older, "magic of this nature always carries a price, Harry. I implore you, do not use your patronus save in a life or death matter until you have mastered occlumency."

Still somewhat in shock, Harry assented; he had not known he was losing the memories he used, and he had no desire to end up a drooling shell like Lockhart.

"Yes sir," said Harry quietly, "I won't use it again unless I have to." He paused, still shaken, and continued, "Erm, sir... where... how do I learn occlumency? Can you teach me?"

Dumbledore straightened, looking Harry directly in the eyes as if searching for something. When nothing happened, the Headmaster relaxed slightly, having apparently gotten the reaction he wished for, and said, "I cannot immediately, Harry, but at the beginning of next term I shall make the time." He smiled softly, "You have my word. Now, I believe I have kept you from a celebration that is rightfully yours."

Still deep in thought, Harry got up and walked towards the door, pausing only briefly upon the threshold as Dumbledore softly spoke, "Be careful, Harry."

Though he was by no means a genius, Harry was certainly not foolish either, and he refrained from casting his patronus; he rather liked his joyous memories intact and accounted for. After explaining the situation to Professor Lupin – who likewise demanded that he cease to practice, and apologized profusely for his ignorance of the charm's lesser known attributes – Harry began to look for texts on occlumency, garnering a few recommendations from Dumbledore himself.

However, Harry's typical luck reared its head, as he discovered in rapid succession Pettigrew's betrayal, Sirius Black's innocence, and the fact that Black was actually his Godfather. After the revelations of the shrieking shack and Pettigrew's subsequent escape, Harry found himself in the unfortunate situation of standing next to an innocent man sentenced to the Kiss while surrounded by dementors.

It was, Harry thought wryly, probably what Dumbledore had meant by a 'life or death situation.'

As the dementors's effects were setting in – the rolling fog of despair and fear settling over his mind far stronger than with a simple boggart – Harry steeled himself to cast his patronus. He forced himself to think of the train ride in first year, of the instant in time when he realized that he had made a friend Dudley could not remove, and shouted, "Expecto Patronum!" A stag bounded from his wand as he felt a portion of the memory vanish, knowing that Ron was his friend, that he could depend upon him, but no longer possessing the moment of that realization.

The knowledge of this loss pained Harry, and, though his patronus was fully formed, it faltered, staggering back from the onslaught of the full might of Azkaban's guards. As the patronus began to fade, two dementors streamed fluidly around it. One approached Sirius immediately and lowered its hood as the second drifted towards Harry's position, likewise lifting its rotted hands to uncover its face. The first bent down over Sirius's prone form, and as Harry's vision began to fade he saw a blinding white orb rise from Sirius's mouth toward the putrid, gaping hole of the dementor's mouth.

In a panic, Harry gathered his mind one last time, focusing his will as much as he could, ignoring the dementor rapidly approaching him. He thought of Sirius, that he was innocent, that Harry would be leaving the Dursleys forever. Knowing that he had only one last chance – one final attempt – to drive away hundreds of dementors to save himself, he focused on hope itself, on happiness and love, while the panic and rage at his situation percolated in the back of his mind, Harry shouted, "Expecto Patronum!"

A blinding white stag erupted from Harry's wand, streaked with silver and black. It was far brighter than a proper patronus, the brilliance of its white body shining with the same ethereal glow that radiated from Sirius's soul. The patronus charged directly into the dementor which was attempting to Kiss Sirius, leaping head-down into its chest. As the antlers connected, the beast stiffened and its floating ceased.

The body of the stag passed through the dementor, emerging as a true mirror to the orb rising beside it, its preternatural white no longer marred. Harry's eyes flashed briefly as ribbons of black and silver oozed along the dementors skin from the point of contact, an acrid smoke whispering up where the strands met. As the foul thing fell – unmistakably dead, its brethren fleeing from the sight – the stag vanished, not dissipating as usual, but rather seeping into the earth.

"Did you hear me?" Ron said weakly, though he was clinging painfully to Harry to stay upright. "You'll have to kill all three of us!"

"There'll be only one murder here tonight," said Black, and his grin widened.

"Why's that?" Harry spat, trying to wrench himself free of Ron and Hermione. "Didn't care last time, did you? Didn't mind slaughtering all those Muggles to get at Pettigrew... What's the matter, gone soft in Azkaban?"

"Harry!" Hermione whimpered. "Be quiet!"

"HE KILLED MY MUM AND DAD!" Harry roared, and with a huge effort he broke free of Hermione's and Ron's restraint and lunged forward –

The world blinked.

He didn't know how he had recovered his wand – didn't care that he was a skinny third year, whereas Black was Voldemort's Right Hand – all Harry knew was that he wanted to hurt the man in front of him to pay for what he had done.

Harry was aware that he did not know much magic; he certainly didn't know how to inflict pain or to kill with it. However, if he had learned nothing else in ten years as a muggle, Harry had learned that magic was not a prerequisite to death.

As Sirius stirred, his soul no longer imperiled, Harry spoke, his tone recalling the recent chill of Azkaban's host:

"Petrificus Totalus."

As Harry approached, vision tunneled onto the man in front of him, he could see Sirius's eyes following him, a strangely confused look seared into the pupils. Standing over the paralyzed man, Harry said quietly, "You killed them."

"You killed my parents, and I will not forgive that."

He kneeled, straddling Sirius's emaciated ribs, and returned his wand to his back pocket. Staring into Sirius's eyes from a meager few feet above, Harry smiled slightly.

"No Ron or Hermione to hold me back now – just as well, I suppose." He paused. "I'd rather they not see this anyway."

Realization briefly flooded Sirius's eyes, followed rapidly by fear, guilt – though Harry couldn't imagine why the man would turn now – and an odd resignation.

Harry wrapped his hands as far around the man's neck as he could, leaning his upper body into the motion to force all of his weight into the traitor's throat. The feel of Sirius's pulse beating through his fingers sang to him, imparting a feeling of joy – of rightness – greater than even Fawkes's lilt. As the seconds passed into minutes, the beat slowed, but the rhythm reached a crescendo played by the animal panic in those silent eyes.

As the last beats faded, Harry's adrenaline receded, leaving him keenly aware of the aches echoing through his body. His ribs throbbed, his head felt like it would burst, and – as his vision began to grey out into unconsciousness – he could feel a dribble of blood run down his forehead.

AN: Thanks to DLP in general and Averis in particular for kick-starting the idea for this. If anyone sees a grammar/spelling/content error somewhere, do let me know. The observant and/or obsessive may not that this used to be 'Chapter 1'. Initially this was conceived as a oneshot, but – after a rather positive reception when I first posted it – I decided to expand it. A second chapter exists. It is utter s***. This plot was never meant to go past Sirius' death, and would have been better left there. As such, this is now marked as a completed oneshot. Sorry, but don't hold out hope for more.