The extract from Secrets of the Dark Arts was copied from wiki/Horcrux.
Trigger warnings will be posted at the end of the chapter for those who may need it :)

Sometimes Harriet awoke to a new day and could hardly breathe. Her limbs would feel heavy yet all too light while her muscles would spasm painfully, as if she were under the cruciatus sure. A cotton feeling would cover her mouth and her eyes would feel as though they were swollen shut. Her scar would throb in time to her heartbeat; a cruel reminder of the curse that bound her long ago to a cold life. Sometimes it felt like she had died long, long ago.
Slowly, gingerly, Harriet climbed out of bed and went to the bathroom to freshen up before climbing quietly downstairs. The living room left no evidence of the night before, where Remus had joined the Potters and Sirius for dinner. It had been a nice evening, filled with adults who reminded her of Mrs Weasley's motherly gestures and Mr Weasley's comical comments. And yet, despite the light atmosphere, the lack of danger, Harriet could not find it in herself to relax. It was all so domestic and serene. Too serene.
Harriet shook herself from her musings as her hand gave a painful tremor. The living room was empty, the beginnings of morning light filtering through the large windows, giving a pink hue to the already cosy room. Against the opposite wall from where Harriet stood was a bookcase, shoved so full of books that Harriet was surprised it hadn't already fallen apart. Scanning, the shelf, Harriet found an odd assortment of books, ranging from muggle fantasy, to wizard children's, to books she would expect from the library at Hogwarts.
Harriet went to turn away but stopped abruptly when she felt a hefty pull from her chest. Stepping forward, Harriet frowned. It was almost as though something was hiding at the back of the bookcase. Something meant for her.
Scanning the room fervently for any signs of the Potters, of danger, Harriet made her way towards the bookcase, carefully removing books from the shelf. Stuffed towards the back, an unassuming, small black book sat. It reminded her uncomfortably of Riddle's diary, without the dark allure that left an acrid taste in her mouth.
Before she could reconsider, Harriet found herself grasping the book. There was no title on the book, nor was there an author's name. Carefully, her fingers shaking slightly, Harriet opened the book to find a neat scrawl, clearly done by hand.
Secrets of the Dark Arts
Harriet frowned. Why on earth would Lily and James have such book in their possession? Against her best efforts, Harriet found her hands begin to shake more violently as a light sheen of sweat erupted from her face. Looking around again, Harriet made sure there was no sign of anyone else awake before she turned the page.
A Horcrux was an object in which a Dark wizard or witch has hidden a fragment of his or her soul in order to become immortal. Horcruxes could only be created after committing murder, the supreme act of evil. The process for the creation of a Horcrux involved a spell and a horrific act is performed soon after the murder has been committed. Given that Horcruxes were precious to those who made them, there were usually protective measures made to prevent them from being stolen or destroyed, such as Counter-Charms and curses. The Horcrux was considered to be the darkest art and the most terrible of all dark-
A creak sounded from above. Quickly, Harriet scrambled to place the book back on the shelf, arranging the books back into place in an attempt to leave no evidence of her morning activities. Scanning the shelf quickly, Harriet grabbed a muggle novel that looked vaguely familiar before opening to a random page, throwing herself on the couch.
Heavy, groggy footsteps sounded from the stairway before Harry's face greeted her with raised brows.
"Mornin'," he drawled, covering his mouth lazily as a yawn escaped.
"Morning."
"You're up early."
"So are you," Harriet parroted before she could stop herself.
Harry merely gave a lazy grin before replying "Touché." He disappeared into the kitchen where soft clanks could be heard along with the boiling of water.
Harriet sat towards the edge of the couch, unsure of what to do. Before she could get up or make a fool of herself, however, Harry came back carrying two steaming mugs.
"Looks like you need it," he said as an explanation before passing the cup to her. To her upmost surprise, it only had a dash of milk and, when she tried it, a strong taste of sugar. Just how she liked it.
As if seeing her surprise, Harry gave her a crooked grin before sitting next to her. "I thought, since you're kinda me from a different universe, that you'd take your tea the same way."
"Thanks."
They sat in surprisingly comfortable silence as Harry woke up and Harriet carefully sipped at her still steaming drink.
"So, are Hermione and Ron your best friends too?" Harry's voice was soft as he looked at her with excited curiosity.
Harriet looked up before giving a small smile. "Yep. Ever since first year."
"Really?" Harry asked. "Even Hermione?"
Harriet frowned slightly before looking him in his green eyes that mirrored her own. "Of course, ever since the troll incident."
"Troll incident?" Harry exclaimed, setting his cup down on the table and turning to face her completely.
"Erm, I suppose that didn't happen to you then?"
"No," he replied frowning. A slight pause settled over the room before Harry huffed. "You can't just leave me with no answers! What's the troll incident?"
"Well," Harriet started, biting her lip hesitantly. "Our Defence Professor let a troll into Hogwarts on Halloween. Hermione didn't know so we went to warn her. We ran into the troll on the way."
Harry frowned before leaning impossibly forward. "What happened?"
"Well," Harriet started hesitantly, before launching into details. When she finished, she realised how carried away she had gotten as she timidly examined his face.
Harry's eyes were wide and his mouth was slightly open in a way that would be quite comical if Harriet wasn't so nervous of his reaction.
"That's bloody brilliant!"
"Really?"
"Of course," a new voice exclaimed from the stairwell, making both Harry and Harriet jump, pulling their wands towards the intruder.
James stood there, hair even messier than usual, in his matching striped pyjamas, looking as though he had not suspected to have wands drawn on him so early in the morning. However, he soon seemed to shake off the surprise as he gave a grin that mimicked Harry's exactly. "Not many fully grown wizards can take down a Troll, much less first years."
Harriet blushed violently at the attention, ducking her head. "Really, I didn't do much. It was all Ron and Hermione."
Both Harry and James rolled their eyes. "I doubt that."
James nodded at Harry's remark before turning to face him fully. You staying for breakfast?"
Harry sighed dramatically before heavily standing up. "I better not. Ginny was expecting me home last night. She'll be even more cross if I miss breakfast."
Smiling, James nodded before turning towards Harriet. "What do you want for breakfast, kiddo?"
Harriet couldn't help but blink in surprise at both the unexpected attention and the question itself. No one had ever asked what she wanted for breakfast before. "Erm, well," she stuttered, "I'm not really picky. Anything's good."
The answering frown from James made her stomach clench uncomfortably. She really didn't want to disappoint her fa- James. However, soon his trademark grin appeared. "Then we'll do some eggs and toast! Cant go wrong there."
As it turned out, eggs and toast could go wrong. Not even five minutes into scrambling the eggs, James had caught them on fire. Harry had been quick to douse it with fire before leaving quickly after, muttering something about "incompetent cooks". Harriet took over after that, much to the chagrin of James who exclaimed that he couldn't have her cooking as that was his job. Harriet was quick to reassure him that she liked cooking.
Not long after the eggs had been finished, Lily came downstairs, already dressed comfortably in jeans and a cardigan. She was quick to help with toast, shooing James away to set the table with a muttered "That man!".
Soon, the small group was sat at the table, food steaming pleasantly in the golden morning light.
"What did you do with these eggs, Harriet, they're excellent!" James exclaimed as he shovelled another mouthful into his mouth.
Harriet ducked her head at the compliment before smiling slightly. "If you add a little bit of milk and cook them on a low heat they become softer."
Lily smiled encouragingly at her. "Where did you learn to cook so well?"
Harriet couldn't help but wince slightly at the subject, slouching further into her chair as if that would make the topic disappear. "Well, my aunt taught me."
A loud clatter sounded throughout the room. Harriet looked up quickly, scanning the room before realising it was just the cutlery on both James and Lily's plate being set down rather forcefully. Both faces were pale, brows furrowed together in conflicting emotions that made Harriet rather uncomfortable. "What?"
Harriet found herself shifting restlessly in her seat mind whirring with questions of where she went wrong. She didn't think she said anything to offend. "Well, er, my aunt sometimes needs help with the cooking so she taught me how to help."
The Potters shared a look at each other before Lily leaned forward, forearms resting on the table. "Do you mean Petunia, darling?"
Harriet furrowed her brows both at the serious face and the term of endearment. No one had ever called her darling before. "Erm, yeah?"
Again, the Potters shared a look before James gave Harriet a rather forced smile. "Well, they're fantastic."
The ensuing silence felt heavier than before, almost like a thick, fleece blanket on a summers night. Suffocating. Harriet found herself finishing her breakfast rather quickly, much to the disgust of her stomach which gave a rather irritated rumble.
As Harriet went to clean her dish, her arm gave a painful convulsion, making Harriet watch in horror as the plate crashed to the floor, breaking it into hundreds of small pieces.
Suddenly, Harriet wasn't at the Potters anymore, but rather a place where such mistakes were not tolerated. Her hands began to shake more violently, sending little lightning bolts up her arms and towards her violently beating heart.
"I'm sorry!" She found herself exclaiming against her tingly numbed lips. "I-I-I didn't mean..."
A hand shot inter her line of vision, making her jump violently away as she exclaimed. "I-I'm s-s-sorry!" She expected a sharp pain to blossom from her cheek, her chest, her arm, but none came. Instead, she was only aware of the frantic cry of her heart and the kitchen that looked too much like that kitchen.
Slowly, her heart seemed to quiet its screams and she became aware of a soft hand slowly patting soothing circles on her back. Harriet looked up quickly to see the concerned eyes of James staring back at her. Going to open her mouth and apologise again, Harriet found that the cottony feeling she had felt this morning had seemingly multiplied with intensity, making her cough uncomfortably.
A tall glass cut through her vision along with the gentle smile of Lily as she held it out to her. Harriet went to grab the cup only to find that her hands were still jerking violently, making it virtually impossible to grab onto the glass. As if sensing her problem, Lily slowly helped her hold the glass, covering her impossibly soft hands over Harriet's and guiding it towards her mouth.
The only sound throughout the room was Harriet's unsteady breathing as she carefully swallowed mouthfuls of water. Eventually the glass had been finished and Harriet finally found her voice.
"I-I'm sorry. I, erm, dunno what happened."
"It's alright," came James' soft reply as he quickly repaired the plate with an easy wave of his wand. "No harm done."
"I-I didn't mean to make such a b-big deal..."
"It's fine, sweetheart," Lily remarked, offering her an arm to stand up. "We don't mind."
To Harriet's continued embarrassment, she felt herself swaying as her head spun dangerously.
"Woah, there!" James exclaimed, carefully grabbing her elbow. Harriet couldn't help but flinch. "How 'bout you lay down for a while?"
"O-okay," Harriet found herself saying, despite the fact that she knew she shouldn't lie down, that she should do something useful, something helpful. She blinked and her room came into vision, despite the fact that she had just been down in the kitchen. Right?
"Lay down for as long as you need to," Lily said, draping a soft, woollen blanket over Harriet's form.
Harriet wanted to say thank you, to say that she was sorry, to say that it would never happen again, but her eyes were too heavy and her lips felt glued together. A calloused hand brushing her hair gently back was the last thing she felt.

Lily and James sat heavily on the couch, fire crackling softly in the corner. Both were caught in a whirlwind of thoughts, making the space feel thick, like syrup or honey.
Lily's sigh broke the silence first.
"What was that, James?"
"Panic attack."
Lily tiredly rolled her eyes. "I know that, but why?"
James roughly swiped his hand across his face, looking much more tired than thirty minutes previously. "I suspect that she's been feeling on edge for ages and the breaking of the plate was the tipping point. We see it, sometimes, when we're interviewing witnesses."
Lily nodded slightly before closing her eyes, resting her head on her husband's shoulder. "She's been living with Petunia. Out of all the people..."
"I know," came James' soft reply. "But we don't know if her Petunia is anything like our Petunia. Just this morning, I heard her and Harry comparing their first years at Hogwarts. Harriet's seems extraordinarily different."
Lily's shoulders shook as she shuddered a breath. "I-I know but you've seen her. She's just so... hesitant, shy. Nothing like our Harry."
"I know. I've seen it too. I do think something has happened to make her so skittish. I'm just hoping it was a one-time event."
The crackling of the fire filled the room at the ensuing silence.
"James?"
"Lily?"
"Where d'you think she got that scar? The one on her forehead?"
James winced slightly. "I'm hoping I'm wrong but the precision of the scar, the placement, makes it seem almost like it's a-"
"-curse scar."
A bird's cheerful song filled the room, lively, mocking.
"Lily?"
"James?"
"I don't think she's bad."
"I don't think so either."

Upstair, the young girl thrashed against the invisible forced that held her in a vice-like grip. She didn't cry. She didn't shout. She had learnt long ago to stay quiet when she fought her demons following her relentlessly, even in sleep's embrace. She had learnt long ago to depend only on herself. Even if the loneliness was crushing. Even if she wanted someone. Even if she needed someone. Desperately.

TW panic attacks, nightmares, anxiety, allusions to abuse, nightmares.