Author's Note: This is an aborted beginning to an answer to Whitetigerwolf's Symbiote Harry challenge. More notes at the end.
Content warning: Abuse, violence, dead Death Eater.
Narcissa wondered whether she might be dying.
She wondered whether she cared. Perhaps that was the bitterest thing of all.
If only Lucius would limit himself to the Cruciatus, she would have no physical injury - whatever the mental harm. But he liked to humiliate her in - Muggle ways as well. He denied her also the use of her wand, so that she could not heal herself from whatever he inflicted. To so demean the daughter of one of England's greatest Houses, rendering her little more than a Muggle captive for his amusement, gave him enormous pleasure.
The only consolation she had, these days, was that Draco had his arrogance but not the stomach for similar cruelty. Perhaps there was hope for the next generation.
For she was lost. Even if she could get her wand, her marriage contract forbade her from harming him. He'd taunted her with that, back in the early days when she still had some fire. It hadn't been so bad as it was now. As she grew more resigned and less responsive to his torments, he'd devised new ones to get the same thrill.
He'd always used some caution, however, until the old guard got back together. Yes, the Death Eaters had arisen once more. Rumors even said their Lord might be returning; Narcissa neither knew nor cared. The Wizarding government was so corrupt that they scarcely needed him to avoid the repercussions of their misdeeds, and Dumbledore was a senile old fool. He'd lost Harry Potter, for Merlin's sake. How did one lose Harry Potter? Even after near to a decade, he had no idea whether the boy was alive or dead...
Not that it mattered to her. She breathed in and out shallowly, trying to make the pain less. Lucius had been in a temper when he'd come back this last time, furious over Amelia Bones's attempts to hold back the tide, and... it had been... memorable. He'd told her, this time, that he didn't care whether she died. He had no use for a dried-up old hag, and some of his friends had young, fresh daughters that could give him strong sons - not just one feeble boy who lacked the spine for bloodsport.
Never mind that she was a young woman by Wizarding standards, and it was Lucius who had only sired one child in his life, despite all his whoring on the side; Lucius would never accept that it was his seed that was inbred and feeble, and he liked them young. Quite honestly, if Narcissa had more energy left in her, she'd want to stay alive for the sake of his prospective next wife. It might give the poor girl, whomever she might be, a chance to complete her OWLs before being sentenced to life as Lucius's brood-mare.
Hope had left her long ago, however, and she seriously debated whether she wanted to continue clinging to life. She could not endure another century of being married to this monster - and she could easily reach that in a Wizarding lifespan, provided no unnatural causes brought about an early demise. Thus either her life would give out before then, or her sanity would. Some fragment of pride preferred it to be her life.
What other option did she have? The contract was ironclad: she could not flee, she could not divorce, she could not disobey, and she certainly could not attack him, much as she'd like that...
As she stared bleakly into the shadows of the desolate room, she noticed that one of them was lengthening.
At first, she thought it must have been her imagination. Her vision was hazy and her head was unwell; perhaps it was a trick of the eye.
Then it extended a long, wavering tendril. Not her imagination, then.
Well, what was it? Narcissa had neither the strength nor the motivation to flee as the shadow oozed towards her. Some Death Eater experiment that had escaped? Some novel execution method Lucius had devised? She watched with woozy interest as it crept closer. More tendrils spread out from its amorphous form, seeming to sniff the air. One prodded her damaged hand, then flattened itself out upon the broken flesh.
She instinctively flinched, but the flinch hurt more than the contact. The thing soaked down into her skin, a warm, comforting sensation spreading with it, and soon encased her whole hand. She could feel the broken flesh knitting itself together.
Trailing off, she watched as it flowed up her arm, wrapping itself around the flesh like a skintight fabric. Everywhere it touched, she could feel it healing her... strengthening her...
And feeding off of her magic as well, she noticed. Well, if it intended to leave her dead or a powerless Squib, it was certainly going about it in a strange fashion - no mere parasite would bother healing its host. No, she rather thought that, however it drained her - whatever it did to her - she would still have some magic.
Thus she was content to watch, fascinated, as it slithered over her body. When it reached her torso, she was treated to a host of grinding, miserable, revolting sensations - a few of which were recognizable as her broken ribs being set back into place. Others, she could guess in retrospect, were from it similarly repairing damaged internal organs. At that instant, though, she could only let out a shriek of pain.
As the horrid sensations faded, she lay on the ground, panting - having the energy to pant, and being able to pant without crying in pain. A soothing pulse went through her as the black substance covered her body, as though this creature were trying to reassure her. It was the nicest thing anyone had done for her in years.
She startled as the creature extended over her mouth, but discovered a moment later that it permitted air to pass freely. When it covered her eyes, she had a few moments of utter darkness, but it soon gave way to translucent patches, allowing her to see. Really, she had to wonder if it was designed to serve - could any natural creature be so accommodating?
First came a sensation like fear, then hummed agreement. Yes, she thought it might be saying. Serve. Wish to serve.
Narcissa stood - when, a scant few minutes ago, she would have been in barely any state to crawl - and looked down at herself. The creature clung to her like a second skin, pitch-black and smoother than silk.
But more than that was the strength she felt through it. She clenched a fist and drove it into a wall.
The fist went straight through, and the impact barely hurt. Surprised, she drew back her hand and inspected it; it did not even look scuffed. Well, of course. It was a magical creature, not truly clothing.
Going to one corner of the room, she sprang towards the other, and found herself there so quickly that her arms barely came up in time to keep her from smashing headfirst into the wall. Perhaps they had a little assistance. She looked down at herself, then, for the first time in a long while, slowly, genuinely smiled.
Then came the familiar pounding of footsteps, and her gaze snapped towards the door - one that locked from the outside, as did most of the doors in Malfoy Manor. "Just what do you think you're doing?" roared Lucius. "How dare do you make that awful racket?"
Ordinarily, if she'd made any sound, she would be the first to fall into mechanical humility and promises of future compliance. Now, however, she merely stood and watched with contemptuous eyes, flexing her coated fingers and wondering whether there might be some out to that benighted contract after all. It was difficult to feel pitiful and helpless with this creature surrounding her - embracing her -
The door flew open a moment later, and contemplation ended. Lucius recoiled when he saw her new self. "What have you done? What -" Then the Death Eater in him overcame any hesitation he might have felt. "Bitch!"
The sickly-purple bolt that flew towards her should have been impossible to dodge.
Which was why she was as surprised as Lucius when her dive to the side left it flying past her with inches to spare. Not one to hesitate, however, she lunged for him as he gawked. Sheer inertia carried her onto him and sent him sprawling to the floor, but she felt the magic of the contract tightening around her throat and arms like iron bands when she attempted to close her hands around his neck. A scream of horror and rage echoed inside her skull. So close, so close -
And the creature finished the job for her.
She watched in shock as tendrils lashed out from her clawed hands and assaulted Lucius's face and neck. Though his own magic tried to fight it off - and never before had she seen accidental magic from a grown man - the black tentacles relentlessly bore down upon him, even as he screamed and battered at them with his hands. For her own part, holding him down was easy with her newly-enhanced strength.
And, when the creature secured a good hold on him, the struggling stopped very quickly. Its treatment of him was nowhere near as kind as its treatment of her: the two were as light and darkness. Blood pooled around his head as muffled screams came from beneath the black mass; the more blood, the weaker the screams. Where the mass separated from flesh, the sight made Narcissa gag. That was only a physical reaction, however - For the man himself, she had no pity.
If his treatment of her had not been enough, his actions toward his other victims would have been.
When the screams stopped and the body went slack, she rolled off of him and watched as the creature feasted. Yes, she thought distantly, hugging one knee to her chest as she observed how the blood ruined Lucius's precious rugs, existence was more pleasant without the contract binding her. It was as though... well, rather as though a large black mass had been weighing down upon her upper body.
She averted her eyes as the creature continued to gorge itself upon her late husband's carcass. She had not really needed to know how little of Lucius's upper body was left.
Whatever this creature was, she owed it her life - a life worth living, at any rate. She wondered what it might want.
It stopped briefly. Again, she felt the humming sensation that, perhaps, might have meant to serve.
Very well and good. She would prefer more details, however. Temperature ranges? Ideal humidity? Dietary preferences? Obtaining more Death Eaters on short notice might be a bit inconvenient, but she would see what she could do. Whatever its likes and dislikes, she owed it -
It stopped again, and this time it stilled for over a minute. At length, she felt more complex impressions from it, though the effort seemed to be a drain on its energy - shortly replenished from her own magic.
Vaguely, the image of a howling, hate-filled monster swam into her mind - one that menaced a huddled, helpless figure. Hurt.
Then came a greater focus on the weak, fearful figure - one with a blurry sense of identification, though Narcissa didn't know whether that came from her or the creature. Help.
"I see," Narcissa said aloud. "Protect the weak and protect against the strong?"
Narcissa sat and pondered. It had been a long time since she had thought of planning the future for herself. She swirled a finger in the drying blood.
There were certainly many of the latter in England. She knew their names. She knew almost all their faces. In many cases, she knew they lived.
She knew England would be better off without them.
"Yes," she mused aloud, as much to herself as to the creature. "Yes, I could do that."
It seemed she would be obtaining more Death Eaters on short notice after all.
Author's Note: Cue long, action-packed sequence of Narcissa slaughtering her way through the ranks of the Death Eaters, culminating in Voldemort returning and having an epic showdown with Symbiote-wielding!Narcissa.
The problem is that I can't write action. The entire point of the oneshot would be nonstop Death-Eater-killing action with a skeletal plot to justify it, so I can't kludge around it by skimming over action and just hitting plot points. Bother.
For further background, the Dursleys were even more aggressive in their abuse than in canon, with their rants particularly centering around Harry being a "parasite". Hammer it into a magical child's head that he's a miserable, pathetic creature that can only exist by living off of others and ought to repay that parasitism through unquestioning service, and it might become true. Mind, the symbiote didn't stay around them after the transformation, because, really, even a formless, bodiless creature has standards.
Several years after the fateful transformation, the symbiote continued to eke out a feeble existence by hitching rides on rats, pigeons, and other such vermin. (The resemblance to Voldemort's Albanian years was unintended, but interesting.) While coincidentally crossing the grounds of Malfoy Manor, the symbiote sensed Narcissa's injury and despair, experienced strange, sympathetic stirrings, and, detaching from the current host, crawled towards the source of those unpleasantly familiar emotions. The rest is history.
My vague thoughts on the non-action parts were that, through sufficient binding to Narcissa and feeding on enemies, the symbiote would regain more of an independent mind, have more autonomy, and eventually be able to construct a humanoid body for personal use (while retaining the ability to revert back to symbiote form at a moment's notice). In short, Harry would gradually reemerge from the "symbiote", but probably not make a full reappearance until a final 'epilogue' scene after Voldemort's demise.
Of course, there's a plotting problem in that, since neither Narcissa nor symbiote!Harry have any knowledge of Horcruxes, Voldemort would just pop up again and again. On the other hand, since Narcissa is becoming a comic-book superhero/supervillain, it would be oddly appropriate for her and Harry to live with a recurring adversary whose death never quite sticks...