Standard disclaimer: Of course I don't own any of this; I'm just playing in someone else's sandbox!
Creating spells was one of the best feelings he'd ever had, even when it hurt, which it often did. It was hard work, like running for miles or climbing a tall hill, and it was draining - emotionally, physically, intellectually. He slept like a child on those nights after he worked on them.
It had taken many hours in the library to even learn how, and learning had been anything but straightforward. It was as if the authors deliberately conspired to keep too much information from appearing in any one book, or even within the books about any one subject.
But painstakingly, he had pieced it together. The mental discipline, the research, the infusing of a single phrase and motion with the power of magic.
He started small, creating variations on spells he already knew. A levitation spell that worked only on the non-organic matter of a given space, for example. He experimented on a patch of ground until the spell could reliably lift every stone and fleck of mineral while leaving the grass and organic soil alone. That had taken weeks of study and work. He thought it useless as spells go - perhaps it might be useful for finding things lost in an overgrown meadow, but he could see little practical purpose.
It had only been an experiment, a learning spell. He was much faster now, and much more adept.
Tonight, he knelt in an empty room, deep in the castle's complex subterranean tangle. He doubted many people alive knew this room even existed. It had been ankle deep with dust when he found it. With its single door, lack of windows, and noiseless qualities, it was perfect for this art of crafting new spells. Perhaps that was even its original purpose.
Around him, he arranged the items that his spell would mimic, murmuring a rhyme as he placed each element:
A single long, black thorn from a tall bush he'd found in the forest. "To represent the earth in which it grew," he whispered.
A mummified, clawed foot from some vicious but long-dead bird, "To represent the air in which it flew," he spoke.
A shard of glass, shattered to reveal its hidden molecular edge, "To representing water, clear and cold."
A steel scalpel, honed so sharp that it didn't even hurt when you sliced with it, "To represent fire which made its mold."
There were many variations, and the creativity of this particular process, of finding the elemental representatives, was part of what drew him to it.
He laid the items out at the cardinal directions, though he doubted that really made much material difference to the spell. Every part of spellcraft, it seemed, was about honing the mind of the crafter. Placing the objects deliberately meant that the mind could find them easily as it needed. Anything could be used to represent any element. It was all about convincing the creator, focusing the mind.
Once the objects were placed, he whispered "Nox," and the room settled into pitch-blackness. This spell would have little uses except for dark magic, and so it must be created in darkness. He placed his wand on the floor, perpendicular to his knees, pointing west, toward the setting sun.
Opening his mind to the magic that seemed to swirl around him, he began the meditation. He repeated the desired action out loud, amplifying it magically until the words bounced around him as a garble, a thousand voices calling out the syllables that would become the verbal aspect of his new spell.
He reached down, and lifted the first vial, uncorked it and poured it in an intricate pattern, weaving it around the objects where they lay at their four points around him, and describing the action the spell would take. As he twisted, the magic turned him, or perhaps the roof spun around him. He wasn't sure how it worked.
"To slice the skin and release the lifeblood into the earth," he intoned, beginning and ending the pattern on his earth element, the sharp thorn.
"To separate the body and sear them apart forever as with flame," he called, his fire element now drawn into the spell, as he poured out the second vial of blood.
He grasped the third vial, and repeated the action. "To open the veins and pour out the lifeblood like a river," he spoke louder now, engaging his water element.
"To release the soul to be blown away with the wind," he cried, completing the lacy pattern with the fourth vial of blood, bringing in the air element.
"To sever, forever," he screamed, and his wand seemed to leap into his hand and he began the motion that would accompany the spell, over and over, training the magic.
His amplified, echoed words crystalized into a single voice, as he called out the spell, over and over, and with each syllable, his own body was carved and sliced, and more of his blood spilled, until he lay in a heap on the floor, pale as death. There was always a sacrifice for creating spells. He had often wondered how the creator of Aveda Kedavra had held onto life long enough to create the spell.
As the words drifted away, he lay in a daze. Sectumsempra was born from his own body and blood.
In his office, the headmaster bowed his head and listened as the castle relayed to him what was happening deep in its dungeons, and sighed.
"Severus," he whispered to himself, shaking his head and feeling older than his years, "Severus, whatever will I do with you?"