Note: This story begins at the end of "His Last Vow", from which point it deviates from the canon. Eurus does not exist in this story (mainly because it's a shitty name and also because Season Four is being entirely disregarded).
Chapter One - 24th of December, 2014
"You just remember it all?" Dr. John Watson asked from where he stood next to a still silent Sherlock Holmes.
"It's all about knowledge." Charles Magnussen replied, getting up from his chair. "Everything is. Knowing is owning."
"But if you just know it then you don't have proof." John argued as Magnussen slipped past him, making his way over to the long mahogany desk.
"Proof? What would I need proof for?" Magnussen chuckled, trailing his hand over the wood. "I'm in news, you moron."
If it were possible, which, apparently, it was, Magnussen's smirk grew even smugger as he settled into the plush chair behind his desk, leaning back and clasping his hands together casually on his stomach. "I'd don't have to prove it. I just have to print it."
"Speaking of news, you will both be heavily-"
At the crack in the man's voice, Sherlock's expression changed from one of frantic recalculation to sudden alertness.
"You will both," Magnussen attempted again, before he reached up, removed his glasses, and vigorously rubbed at his eyes. "You will both be, be featured tomorrow with, with state secrets and,"
His glasses dropped to the carpeted floor.
"What," Magnussen's voice cracked again, this time as his head lolled frighteningly to one side. "What have you done to me?"
Sherlock and John exchanged questioning glances. Magnussen gave another lurch, prompting John- ever the doctor- to rush forward, searching for the man's pulse along his jugular vein.
"Sherlock, his pulse is erratic; he seems to be going into-" John cut off as Magnussen pitched forward, landing heavily upon his desk.
"Cardiac arrest." A feminine voice finished, floating through the opened doors of the balcony directly adjacent to the three men. "But not for another ten minutes or so."
John whirled around just in time to see a rather small figure drop down from what must've been a small roof or ledge above the doors, landing in a soundless crouch before gracefully righting herself at a height no more than 160cm.
"Tell me, Charles, does it hurt?" The woman stepped forward, lifting the shadows that had previously hidden her features. Lit by the synthetic light of Magnussen's office, John could now see the pomegranate-colored tresses that fell just past angular shoulders, a light scatter of freckles across a small, pointed nose, and gray-blue eyes that shone sharply from under thick eyelashes. She looked rather familiar, though John couldn't place why.
She was no older than twenty-three or twenty-four, and dressed in a grossly oversized blue cable knit sweater that threatened to engulf her knees. On her feet were tactical boots, though her pair lacked both general bulk and heaviness of the soles, and on her legs were thick leggings printed in black, blue, and grey camouflage.
"Or is it only the paralysis that's begun?" She asked, and then moved towards them, the swishing of her wool sweater the only detectable sound.
Reaching the side of the desk, she stopped, leaning down to study Magnussen's head where it lay in a heap of disturbed documents. Drool had begun to trickle out the side of his mouth.
"I suppose you're hardly in any shape to answer." She considered, and then looked up. "Perhaps, if you'd not pissed in his fireplace, Dr. Watson would be more inclined to help you right now."
John, startled at the mention of his name, looked over at Sherlock, who had adopted the confused, if not altogether lost, look that had stricken his face just moments earlier when Magnussen had revealed his vaults- or distinct lack of- to the two.
"You know," She continued. "You two aren't supposed to be here. Mycroft said he'd keep you occupied."
Sherlock's eyes flitted, first to the incapacitated form of his most current archenemy and then to that of the most recent arrival, before finally landing on John. Noticing the subtle way John's hand was beginning to migrate to the back of his waistband where both men knew John's army revolver was securely kept, he shook his head sharply. John's hand stopped.
He turned back to the woman. "Yes, I drugged him." He replied, a touch of forced nonchalance seeping into his tone.
"Wonderful." The woman laughed lightly, albeit dryly. "I'm sure that'll make for a lovely Christmas."
"Sorry, but what's-" John began, his forehead crinkled in confusion.
Sherlock quickly interrupted. "John, this is Seraphin. Seraphin, this, as you clearly already know, is John."
The woman, Seraphin, smiled, leaning over the desk with her hand outstretched in greeting. "It's nice to finally meet you, John. I've heard a lot about you, of course." Hesitantly, John shook it, still glancing at Sherlock questioningly.
Seeing this, Seraphin sighed. "Really, Sherlock. You've lived with John for how long and you still haven't mentioned me?"
"I wasn't planning on mentioning Mycroft either, but he took the initiative." Sherlock shrugged dismissively, looking pointedly away from both accusatory sets of eyes.
"Your," John, not being nearly as slow as most people credited him as being, connected the dots. "Are you Sherlock's sister?"
"The youngest and least aggravating of the Holmes siblings, yes." Seraphin smiled broadly, exposing a row of white teeth and particularly pointed canines, before leaning over and replacing John's fingers at Magnussen's pulse point with her own. "But there will be time for more thorough introductions later. For now, I have a job to do."
"A job?" Sherlock questioned.
"Yes. It seems, my dear brother, that the powers that be found you in need of intervention." She hummed in response, moving quickly behind the desk and leaning over Magnussen's prone form. "Again."
"And by powers that be you mean Mycroft."
"Of course."
"And you're his intervention?"
She nodded absently in agreement as she carelessly lifted Magnussen's head by his right ear, examining his pupillary response and sclera with interest.
"I was fine on my own." Sherlock continued, his lips turning downwards in annoyance.
"Were you now?" Seraphin looked up sharply, dropping Magnussen's head with a dull thud and an answering groan. "Don't play me for a fool, Sherlock, we both know I'm not one."
Sherlock scoffed, but she continued to examine Magnussen with only a somewhat cross look in her brother's direction.
"Right now, you're desperate. Magnussen's files have proved intangible; his vaults, non-existent. Your plan to entrap him through Mycroft's laptop has backfired, placing both you and John in a rather precarious situation. He's beaten you. And so, you're left with only one option."
She paused, as though to give Sherlock the opportunity to chime in. Seeing him in brooding silence, she continued. "That one option is to kill Magnussen. It makes sense, I suppose. After all, you have the opportunity, the motive, and the tools. But you're missing something rather important."
"And what might that be?" Sherlock's eyes narrowed in challenge.
"The escape." She answered simply, before curving herself around Magnussen's fallen form, carefully avoiding any bodily contact with him as she extracted his cell phone from his breast pocket. "But of course, you know that."
John's eyes were now frantic as the bounced between Sherlock, Magnussen, and Seraphin. "Sherlock, you can't-"
"Quiet, John." Sherlock snapped.
"You plan on getting caught, don't you Sherlock? Because in your mind, the consequences- which, may I remind you, could range from your permanent termination to exile or imprisonment- are worth it. Mary will be free, and by association so will John, and they'll be able to live out their happy, boring little lives together in a house surrounded by a white picket fence with a baby and maybe a dog." She snorted contemptuously as she popped open the back of the phone, removed the battery and the memory card, replaced the former, and then settling the phone back in Magnussen's pocket. Flipping back his suit jacket, she pulled a small silver key from a hidden pocket that had been sewn into the lining. "You've gone soft, brother mine."
She moved towards a locked drawer in Magnussen's desk. "I suppose love does that to a person,"
"Shut up, Seraphin," Sherlock began, teeth clenching so Sherlock's teeth were clenched so tightly that John could hear them squeak.
"Even so, what a disappointment." She finished, ignoring the agitated man as he slammed his palms onto Magnussen's desk, heedless of the man who lay there.
"You know nothing."
"I know everything, Sherlock." She replied, holding his angry gaze easily.
"You, on the other hand, apparently lack the barest of common sense. I mean really," She laughed, chastising, as she pulled an ornate wooden box out of the previously locked drawer. "Have the barest of faith in Mycroft and all the other higher powers. After all, they didn't let Moriarty dethrone them and he was far greater a man than Magnussen could ever hope to be."
"Please, they were perfectly content to let Moriarty prance about London playing God so long-" Sherlock began, before being cut of as Magnussen gave a rather desperate gasp. Seraphin glanced at a small, leather bound watch on her right wrist.
"Respiratory distress at minute twenty-eight. Right on track, then." She grinned, before dislodging a satchel that had sat previously on her back. Carefully, she slipped the box and the memory chip into it and then placed the bag gently on the ground.
"Not that it really matters now, but how were you planning on dispatching him, hmm?"
Sherlock remained stubbornly silent, though his eyes betrayed ambiguous curiosity as he studied the gasping man beneath him.
"A gun, perhaps? It'd have to be John's, seeing as how Mycroft has been quite religious in sabotaging your attempts to get your own. You brought a knife with you- in your back pocket- but that's awfully messy for you- probably only brought along a last resort, then. It's possible, and considering Charles' limited agility and experience and slightly shorter stature, that you planned to best him in hand-to-hand combat, as it would be a fight easily won. But no, I think I'd have to go with the gun." She cocked her head curiously, drawing closer to Sherlock as she moved out from behind the desk.
"That seems rather out of character for you, Sherlock, just killing him. You didn't even try to come up with another cleverer option, didn't try to outwit him at his own game. Instead you were going to just," She made a gun with her fingers, pulling the imaginary trigger. "Kill him. It's all rather dull."
"Isn't that what you're doing?" Sherlock shot back, his grip tightening on the edge of the desk, turning his knuckles white.
"Ah, but you're missing the nuance, Sherlock." Seraphin sung, shaking her head and causing a tumble of red curls to drift into her face. "This isn't killing him. This is destroying him."
"Look." She motioned towards Magnussen. His eyes were wide though not unseeing as his mouth opened in a voiceless cry, his body trembling and shuddering, chest heaving.
"What you can see are only the physical effects. First, his nervous system failed, causing general paralysis of the body, while keeping his mind intact. He would've felt some symptoms beforehand, of course- the quickened heart rate, the sensation of an impossibly dry mouth, and perhaps even some dizziness- all of which he disregarded in favor of chatting with you.
"After total paralysis, he would begin to feel a heat right in foremost of his brain. Right behind the eyes, or perhaps around the temples. A bit like a migraine. He likely wouldn't have noticed this, given that paralysis is generally more concerning to most than a silly headache. But then, the burning would've started.
"I imagine it would, at first, feel as though there was a hungry little flame sitting atop your thalamus. Then, it would radiate outwards, as though someone had set fire to your brain from within your skull- encompassing your cerebral cortex and occipital lobe and perhaps even the vitreous body of your eyeballs, eating away at your tissues, cells, and nerves like a ravenous little hookworm.
"That would hurt quite a bit, but it wouldn't be what hurt the most." She leaned down to meet Magnussen's eyes, now locked on her form and seeming to cling to every word she was saying. "What would hurt the most is him having to watch as his archives- each file individually filled with pressure points and weaknesses and personal data all so meticulously collected and tucked away to be retrieved at any time to suit any purpose- burned to ash. Perhaps he'd try to save them, burning his hands as he so desperately attempted to stop the fire from ravaging them, only to fail. He would find he could do little more than watch as his vaults were torn down around him, burying him in soot and decay. His precious mind palace- gone."
Magnussen had begun to seize properly, his body clenching and retracting against the desk, the chair threatening to fall out from under him. "Even now, as his body goes into shock and his internal organs begin to shut down, he can't bring himself to care about anything else but the ruins.
"I bet he'd have preferred it if I let you shoot him, Sherlock." Seraphin rummaged in the desk one last time, before producing a small flash drive. Instead of placing it in her bag along with the box and memory card, however, she merely dropped it to the floor and, with a sharp, downward movement of her foot, crushed it underneath her boot. "It'd have been much easier."
A frothy mixture of blood and saliva had begun to pour from the side of Magnussen's mouth.
"That would be his heart failing in the thirty-first minute." She said, picking up a loose pen and slowly trailing it through the fluid, further distorting the documents that lay under him. She then met the man's eyes with an almost kind smile. "You should be dead within the next two to four, if it's any comfort."
She dropped the pen into the bin beside the desk, pointed nose wrinkling in vague distaste.
The sound of an approaching helicopter could be heard amongst the gurgling sounds of Magnussen's mangled, quickening breaths, prompting Seraphin to look up.
"That's my cue, I'm afraid." She sighed, rolling her shoulders and flexing her neck from side to side.
"But before I leave," She trailed off, instead rushing soundlessly from the room in the direction the men had come.
John quickly turned to Sherlock, clearly bursting with numerous yet-unanswered questions.
"It's," Sherlock began, haltingly, before shaking his head. "I'll explain later. We haven't the time now anyways."
Seraphin re-entered, this time carrying Mycroft's laptop. She quickly made her way over to her satchel, slipped the computer inside, zipped it back up, and slung it across her back. "I'd better be off, Mycroft's men will be arriving any moment."
Sherlock nodded- the sounds of the helicopter blades beating against the air were much closer now, likely mere seconds away from landing.
She rushed towards the open doors, one leg up on the balcony railing before she paused. "Oh and Sherlock," She said, glancing back towards the two men. "Do hurry home. You know how Mummy so enjoys Chrismas morning."
Without another word, she leapt, disappearing into the cool night of the Cotswolds countryside.