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"After all, He-Who-Must-Not-Be-Named did great things — terrible, yes, but great."
Ollivander, Harry Potter and the Philosopher's Stone
Severus creates the spell in a frenzy of loathing and fury and vengeance. Sectumsempra is his crowning glory, the capstone to his achievements in the realm of spellcrafting. No other cutting, slashing, ripping curse results in quite as much agony and permanent damage as his spell.
‘FOR ENEMIES,’ he writes with a flourish in his textbook, glaring daggers at the so-called Marauders across the room. He imagines flinging the curse at Black the next time the mutt tries to corner him after lessons. The pained screams and begging would be music to his ears.
When his book goes missing, gets stolen, he immediately turns to the Marauders. Their smug grins and taunting looks tell him enough of their culpability. His wand arm twitches, his new spell on the tip of his tongue. Then a gaggle of Ravenclaws walk by and he slinks back into the shadows, opportunity lost. He will find another chance.
News spreads like fiendfyre of a girl, a muggleborn, abed in the infirmary with a severed arm, separated so cleanly at the shoulder joint it was as if it was by design. Dread fills him at the thought. He sneaks into the hospital wing, but he doesn’t need the confirmation.
He knows that it was his spell that caused the damage.
The matron cannot do anything for the girl. A third year. Hufflepuff. Whose only crime was being born to parents with no magic in their veins. She is transferred to Saint Mungo’s still bleeding profusely.
Damn the Marauders for stealing his book. Damn the Slytherins who found it chucked deep in Salazar-knows-where. Damn himself for creating that thrice-accursed spell in the first place.
He throws himself into work, researching healing charms, rituals, potions. Any and all which could possibly counteract his depraved mistake. Armed with scrolls upon scrolls of possible cures and a carton of potions, he locks himself in an abandoned classroom one Hogsmeade weekend and steels himself.
The flesh of his thigh is pale, evidence of too much time spent in the dungeons, too little out in the sun. A pristine canvas. Not for long.
He jerks his wand mercilessly, a single word running through his mind.
‘Sectumsempra!’
To sever always.
A line, no longer than the length of his hand, erupts crimson across his skin. Life blood oozes sluggishly out of the gash. Good, he avoided the artery. The wound is larger than is required for his testing, but he ignores the part of him that whispers that it should be more, deeper – you deserve it you worthless bastar-
He takes a deep inhale and breathes out the pain.
He picks up the first scroll and starts incanting spells.
Episkey.
Medicor.
Confervo.
Not a single one has any effect on the wound.
He has to stop a few times to take a Blood Replenisher. His whole leg is dripping in blood by now. There is a slowly growing puddle around him.
He moves on to rituals.
Rituals of health, of healing, of wholeness.
Blood flow begins to slow. He is not sure if it is the rituals or his dropping blood pressure.
This is your penance, you monster. This is nothing on the arm little Hillary Tannings lost.
He takes a double dose of Blood Replenisher.
And continues.
Next are the potions.
Potions he carefully adjusted and edited to extract the highest amount of healing factor. Concoctions of his own creation.
None give a result.
He has exhausted every single option he thought of. The only way now, he knows, is to create a counterspell. It is something he hoped to not have to come to, for they are notoriously difficult to craft. It is not simple to develop a reversal of a magic spell. Complex theories of magical current interactions and reactions can cause many unpredictable outcomes.
The still-bleeding girl in Saint Mungo’s is an urgent motivator.
As is, he allows grudgingly, his thigh, which is in a similarly sorry state.
He spends the following three days heavily bandaged, downing nutrient solution and replenishers like he would water. His alabaster skin pales even further and he grows ever more reclusive. He skips whatever lessons he can, spending every minute of his free time coaxing apart magical threads only to attempt to knit them back together in an exactly inverse state.
On the third day, running purely on fumes and sheer stubbornness, he manages to get the magic to coalesce into a pale light of pure healing force. His instincts guide the words from his lips, his hoarse voice singing the incantation.
It sounds like salvation.
Vulnera Sanentur.
To heal wounds.
Simple words, yet the magic rushes to the gaping cut in his thigh, stitching up the raw flesh and sealing the wound for the first time in two weeks. A deep scar is left in its wake, a stark reminder of Dark Magic used.
Never forget.
Before he can rest, however, he drags himself up to the infirmary and surreptitiously leaves instructions for the cure.
It doesn’t stop there, of course. His spell spreads through the ranks of Deatheaters. He doesn’t know who exactly is the one who leaked it in the first place. Every person thereafter felled by mysterious wounds that never stop bleeding is another one on his conscience. Before he even graduates he is responsible for the severe maiming and even death of countless people.
He never stops looking for the book. It’s a pointless exercise, because even if he did destroy it, it wouldn’t stop the use of the spell. It’s something he needs to do, however. He needs to destroy it at the root of the source. (He has to stop from regarding himself as the cause of it all, because that way lies only ruin.)
He never does find it.
Until…