"As to how I know about Lightning curses: This should provide a general explanation." As Snape spoke, he tapped his left wrist with his wand, reversing the enchantment that he meticulously kept on his sleeve. The buttons on the cuff slid open immediately and he pushed both robe and shirt away to reveal his Mark.
Pendragon leaned forward to examine it, apparently fascinated rather than repulsed. "I've never seen one in real life before," she commented. "It must be, what, ninety years old? Strange that it hasn't faded or blurred over time, isn't it?"
"Closer to seventy, child. I am not that old. But it isn't strange at all, for this Mark was made with Lightning, though the maker of it did not realize the true nature of the magic he wielded. Fortunate for all of us, I suppose."
"You were his servant. Why?"
Snape waved his hands brusquely. "Another time, Pendragon. I don't know how long this potion will work, and there are more important things to discuss than the motives of a young fool threescore and ten years ago. I want you to understand what I am attempting with you."
"Fair enough," she said, in mimicry of his own words earlier that day, and settled back in her chair.
"The Dark Lord spent decades searching for powerful Dark magic. He held the Rubra Fulminis without recognizing it for many years, using it to Mark his followers, among other things. It gave him the power to create multiple Horcruxes, which had not been attempted in recorded history. By the time he learned the true nature of this tool, he'd become habituated in its use. Complacent, if you will--he'd become so accustomed to using it in the limited ways he'd devised that it didn't occur to him to simply apply it further. The Red Lightning, after all, is the weakest, the lowest in energy of the spectrum. Black Lightning isn't really black, it's ultraviolet. But I digress.
"Once Lord Voldemort realized that he was trafficking in extraordinary magic, he began seeking its more powerful counterparts. It turned out that his ancestor, Salazar Slytherin, had mastered Green Lightning and was striving towards Blue by the end of his lifetime. That little fact has been rather conveniently left out of Hogwarts: A History, as well as that abominable Professor Binns's syllabus (may his soul please rest in peace at some point). We can't have our beloved institution tainted by the fact that one of our Four Founders was Darker than Asahel Durmstrang, can we?" Snape scoffed in disdain and paused briefly to help himself to some rainwater from her barrel.
"So," he continued, settling back into the chair, "Voldemort confiscated Slytherin's many diaries and notebooks from the Chamber of Secrets, and eventually understood that the Lightning curses were not myth. He was astounded to discover he'd been operating one of them for years, having learned the incantation from a recluse in Tibet or some such place. He was eager to continue Slytherin's research on the Blue, for immortality was his ultimate goal, and obviously Slytherin was unable to achieve it. Red, Green and others were therefore beneath him.
"That was how I came into the ranks of the Death Eaters, child; the Dark Lord needed a scientific mind to trace the Lightning curses, and I arrived in the right place at the right time. Fortunately, I had an opportunity to... rethink my position, before making any significant gains. By the time he returned from his first fall, I was quite determined that he would never have them. Unfortunately, the best way to ensure that was to master the spells myself."
"Now wasn't that convenient?" Pendragon grumbled, but he silenced her with a cool glare.
"If you can devise a better plan, child, do let me know, but I was in a rather precarious position--a double agent, assigned to this research with an expectation for results, under penalty of torture! 'Know thine enemy,' they say, after all. And I was not alone in that decision; Albus Dumbledore took on that burden with me, that we could keep each other in check.
"I learned how to wield it, child, the Blue Lightning, though I have never used it. Some of the instructions were encoded within one of Voldemort's own Horcruxes, unbeknownst to him. Albus chose to destroy the object using the Blue, to ensure that no trace of the message remained. I was present when this was done, and the power was... quite impressive. I can only imagine what it was like when that idiot Wormtail cast the Black."
"It was horrible," Pendragon said quietly, almost whispering. "Beyond horror. It was just utterly wrong in every sense; it made me want to run screaming, freeze in terror, and erupt with rage, all at once. I wanted it to kill me, because I couldn't stand to be in a world where such things could exist. I laughed when that little man exploded. I'm not like that, Snape. But I was thrilled to watch him die for bringing such a thing into reality. And I didn't even mind that my arm was gone--because it saved me the trouble of cutting it off myself. It would never be clean again after the touch of that magic."
She sat up suddenly at the edge of her chair. "Snape! It did so much more than that! This is the first time I've been able to think straight in years! What has it done to me--and how did you stop it?"
"Sit back, child," he admonished, noting the hints of panic in her eyes. "Already the potion's effects are waning. I will tell you what I can, while you can listen.
"There is no remaining description of the mechanism behind the Atra's effects, but I found an ancient text that described the Violet Lightning. It spoke of dissociating self from mind, and mind from magic. Abstract terms which sounded like so much blathering, but I read them carefully at the time, and now I am glad of it.
"It's rather amazing what sort of cast-off, esoteric material ends up in a prison library, child. Books that sit unsold on shelves for years in the secondhand shops. Books long forgotten in attics and cellars, rediscovered by heirs hoping they will be worth something, and finding to their disappointment that no one wanted them in their grandparents' day and no one wants them now. But a man with fifty years of empty time on his hands will open any book, and in dry, unappreciated "psychiatry" texts, I discovered some familiar themes.
"If you were a Muggle, you would long since have been labeled 'autistic,' or perhaps 'schizophrenic.' And such labels would not be entirely inaccurate, for they have been applied to the symptoms Muggles show when remnants of the Lightning curses propagate through their ranks. I spent many years making that deduction, and it is admittedly a hunch, but I believe it to be accurate.
"When that curse touched you, Pendragon, it fractured your mind and your magic into separate shards, only some of which are still able to function without the others. You lost some of your magic, but kept some. You lost your name and identity, indeed, so strongly that it was taken not just from you, but the world as well. Parts of your memory, parts of your personality, parts of your very presence were walled off. In a way, you've become your own Horcrux, for the splitting of the soul is accomplished in the same way. In your case, however, the missing pieces are still within you, just... inaccessible."
"My name... my magic. They're all still inside me?"
"Yes, child. The proof is right here before us--just this one potion has restored some connection or balance in your mind."
"But you did it before just by squeezing me!"
Snape nodded. "Something I read once, by a Muggle with autism. She described how sensations often overpower her, rendering her unable to think clearly. Being squeezed and blinded to the external world relieves the anxiety, allowing her to make internal sense again. I thought it might work for you, and it did."
Her shoulders sagged. "Perhaps I need to move into a very small box."
"That might help," he agreed sarcastically. "A rather impractical strategy, however. I think I can devise potions for a longer-term solution... for a price."
Her eyes were definitely getting wilder, but she was still coherent enough to understand his statement. She furrowed her brow, tilting her head in confusion.
"And what price is that, Professor Snape?"
The words were spoken with cold suspicion, but not by Pendragon. Snape turned to regard the owner of the voice.
"That is a matter between myself and the young lady, Professor Potter."