Disclaimer: I don't own J.K. Rowling's universe, I just like to do really evil things with her characters, on occasion.
Chapter 13 - Changes
Memory, Theo reflected, was truly a subjective and mutative thing; the same series of events, viewed by two separate people, varied widely in the retelling, altered by the basic framework of one's beliefs and knowledge. Hermione's memories were carefully ordered, her entire mental landscape reminiscent almost of a library, which made his task that much easier. Tracey had broken down her defenses enough that it took no effort at all for him to slip past what was left, though he was careful in his editing of her life to date. His purpose, of course, had been perfectly sound and logical: to leave the memories of events themselves intact and merely change her perception of how the events had occurred.
It was a time-consuming, painstaking process, as he scrutinized and analyzed and categorized and sanitized her past. The spellwork wasn't the heavy-handed Obliviate the famous Gilderoy Lockhart had once accidentally used on himself; Theo wielded the charm like the finest of artist's brushes, making only miniscule changes while leaving the rest of her mental landscape intact. He was careful to tie up any loose ends caused by the alterations, and by the time he'd finished the groundwork had been laid for the network of layered compulsion charms; these took less time, mostly because he'd already planned beforehand which charms to weave into the subtle net he left in her mind.
Once her loyalty to him was assured, he turned his attention to the most crucial task of his plan: unlocking the true potential of her magical skill. This was his most closely guarded secret, something his grandfather had hinted was possible but had never had the chance to elaborate upon; it was Theo's father who had inadvertantly pointed him in the right direction and sent him questing through the annals of the family's history. What he'd discovered had shattered his preconceived notions of how magic worked, dissolved the building blocks set in place by the Hogwarts curriculum, and set him on the path to becoming who he was. Ancient journals and records dated before the school's founding had discussed the importance of control, but in an entirely different light from the rote memorization of pre-approved spells that he'd been taught at school.
The dangers involved in unlocking an individual's full magical potential had been one of the reasons Hogwarts had been founded in the first place; such experimentation had proved deadly on more than one occasion. It was why the Hogwarts teachers spent so much time emphasizing the trio of components needed for successful spellcasting: movement, incantation and intent; older students were taught to internalize the incantation, to think instead of speak, but the basic principles remained the same, a rigid cage to direct and limit the powers of the average witch or wizard. Theo was breaking all kinds of rules in the course of following his plan, but it was necessary to achieve his goals; if he occasionally broke the law outright and strayed into questionable moral territory, he justified it as being part of the plan and didn't lose any sleep over it.
By the time Theo was finished, he was exhausted but elated, and he sat back with a tired sigh, dropping his wand. It was too soon to put his work to the test, but he was confident that he'd succeeded in what he'd set out to do. He ran through the rest of the steps in his head, reviewing again the edited version of the encounter in his office: she would remember coming to him the day after the ball, disturbed by her friends' most recent dismissal of her research. It would parallel several other discussions he'd planted among her memories, establishing the pattern of trust; her new perception of his plan had her volunteering to help him, playing on the Gryffindor tendency for saving the world and turning it to his advantage.