Leta Lestrange sometimes wishes she was a better person. She makes excuses for herself: She's a Lestrange, selfishness runs in the family. She's a Slytherin. She's spent years being told that cunning, ambition, power, and purity of blood are traits to be prized above all others. Everything good in her life has come at a cost to someone else. She thinks she should feel bad about that, but she – well, she just doesn't. Manipulating people gives her a bright, sharp feeling of satisfaction that runs all the way through her body. The students at Hogwarts have heard things about her family. They skirt around her in the hallways. They walk away when she tries to join in their conversations. They shoot jinxes at her when the teachers aren't looking. They say she's mad, that she'll end up in Azkaban like her father. So when something bad happens to one of them, it makes her happy. But occasionally, just occasionally, at times like this, she wishes she wasn't built this way.

The thing is, she's fond of Newt. As fond as she is of anyone. Newt has never tried to curse her when her back was turned. The other students avoid him in the hallways too. Even the other Hufflepuffs find him annoying, and Hufflepuffs are meant to be accepting. They think he's odd because he's not good at saying the right thing, or acting the right way, or being interested in the right things. They laugh at him when he practices Hippogriff calls in the grounds. They make nasty comments when they see him emerging from the lake, blue-tinged and covered in weed. They see him laughing on the sloping lawn, surrounded by birds that have flown to his side from the forest, and they suggest his mother slept with a beast to create him. And Newt just lets their cruelty roll off him, never seeking revenge, although Leta knows he could. He's perfectly capable of luring someone into the forest to be trampled by centaurs. He could easily train something with sharp teeth and flesh-dissolving venom to attack. Leta is certain the Giant Squid would pluck students from the lake-side and drown them if Newt instructed it to. But Newt never does that. He never even sends a relatively harmless jinx in the direction of one of their schoolmates unless they start it and he feels there is no other way.

To make matters worse, Newt has come back from the summer holidays suddenly absurdly handsome. In the break between sixth and seventh year, his fragile face has become more manly, the rest of his features catching up with his mouth. When he'd smiled at her on Platform Nine and Three Quarters, it had done something unexpected to her insides. He was taller than when they'd parted, all wiry muscle, and he'd been dressed in muggle clothing, the blue of his shirt standing out amidst a sea of black-robed witches and wizards. He'd been so happy to see her.

Now, when she looks at him, she gets a strange, heavy feeling in her gut. Her chest is cold and hollow. There's an aching in her throat. Sometimes, when she's planning, his image floats across her vision. He's smiling kindly at her, the way he always has, and then his face is falling and he's walking away without her. The look in his face is distracting. It's the same look he had when they found out his cat wasn't a cat and were going to kill it. Professor Dumbledore had intervened then. He'd argued on Newt's behalf, reasoned with the other teachers. He'd saved Felix's life. Newt had always been stood by him, since then. But after Leta's plan comes together, no amount of intervention will save Newt. Newt is collateral damage, and for the first time in her life, Leta feels guilty.

Not guilty enough to stop.

The problem is that Newt is just so easy to take advantage of. He'll do anything for her, because she is the only student in this whole school who has treated him like a friend. She'd picked him out when they had first started Hogwarts, as someone who would be useful to her. Lestranges didn't have friends. They associated with three types of people: Powerful patrons, impressionable followers, and valuable tools. Newt was different and lonely, and lonely people are easy to get at. So she'd chosen him as a follower, and come to discover he was cleverer than he seemed, more powerful than people expected, and extremely skilled at anything he felt was worth his time. He wouldn't quite fit into the follower mold, so she'd made him a tool. Now, after nearly seven years of bright smiles, almost-real laughter, and interested, accepting, vibrant discussions about magical beasts, Newt will give her whatever she asks of him, and want nothing in return. And he'll never see what she's doing, because he doesn't see who she is inside. He only sees the kind, caring, lonely girl who offered him friendship when no-one else would. The one who doesn't exist.

She pushes aside the image of Newt's devastated face, ignoring the tiny voice that says he won't forgive her this time. Of course he'll forgive her. He always forgives her. He forgives her every time she leaves him to take the blame for something she's done. He forgives her when she copies his homework, and when she says something harsh without thinking, and when she forgets to put on her mask and jinxes someone just because it's funny. He wrote to her every week over the summer holidays, even though she never wrote back. She comes to him with tears in her eyes and words of false regret, and tells him how she's just trying to be someone her mother will respect, or that she just wanted to make the other kids laugh at someone else for a change, or that she's worried her blood will turn her mad. And every time, he looks her in the face for a moment, pauses, and nods, and she knows she's forgiven as long as she tries harder to be kind.

In her head, Newt won't look at her. His eyes are on the ground and his shoulders are hunched. He stares at the ground near her left foot, like she's a stranger. Like she's nothing to him.

It has to be done. There's no going back now. Leta's revenge is will be glorious. Bacillus Black will forever regret he made an enemy of her. Nobody humiliates a Lestrange as he did and gets away with it. He is going to pay. Bacillus Black is going to hurt for what he did, and so is every single person who laughed. Glee tingles through her as she imagines his face. He screams and begs and cries. He pleads for someone to call the thing off, but nobody can, because it's furious and wild like Leta, and nothing can stop it. Black will see her smiling face and claim it was her, but no-one will be able to prove it. Everyone will know never to cross her, because Leta Lestrange is brilliant and cunning and terrible, and no-one can ever touch her. A laugh bursts out of her, wild and manic. Soon they'll know to fear her.

The pesky little voice cuts into her joy. What about Newt, it asks. Newt will be blamed. He'll be expelled. Maybe he'll even go to prison. Everyone will know the beast is Newt's. Who else would raise something this dangerous? People will see him trying to calm the beast, to call it off, to stop it, and they'll think he set it on Black. Newt's odd. The whole school knows he's obsessed with magical creatures. Everyone had seen him waiting for Leta, flowers in hand, before the Yule Ball. They'd seen his crestfallen face as she'd walked past him with Black, never sparing him a glance. They'd seen him come to her rescue when Black had been so cruel to her, in front of so many people. Bacillus and Newt had almost had a full scale duel when Bacillus had turned his wand on Newt. Newt had been defending himself, defending her, simply holding his own, but that's not how people will remember it. People will only remember slashing wands and flashes of light, and they'll think Newt had attacked because now a beast is attacking and a beast means Newt. Newt is the perfect scapegoat. He has means and motive and everyone is predisposed to blame him. Something hurts behind her eyes. She hits the wall angrily. He's ruining her fun again.

She stops copying Newt's homework. She says kind things to him and brushes his hair out of his eyes and stops making the kind of jokes she knows cut him slightly. She fetches things for him without being asked, covers for him when he's in the forest. She shakes hands with the giant squid, and laughs when it picks her up, even though she wants to scowl and snap at Newt for letting it ruin her robes. She goes with him to see the unicorns in the forest, and this time she really does marvel at their beauty, where once she had only seen profit. She smiles at him, and talks about how when school is over they'll travel the world together. She stops arguing with him about the pointlessness of Muggles, because it makes him happy to think she understands. He looks into her eyes and smiles. One day, as they sit beside the lake, he awkwardly stammers out that he's glad she's his friend. Her stomach flips.

She lies awake at night, berating herself for her own weakness. None of this will make up for what she's going to do. If she was truly strong she wouldn't care that she was going to ruin his life. After all, she only needs him because he's useful. She bangs her head on her pillow and tries to concentrate on Bacillus Black's screams.

Black calls her ugly names in the hallway and charms up a wind to lift her robes over her head. Everyone laughs. Her resolve stiffens.

On the day of her plan, Newt is happy. His creature let him touch it last night and didn't spit fire at him. He thinks it will let him fix its hurt wing soon. Leta eats breakfast with him, then lies and tells him she has detention and can't go to the quidditch match with him after all. For a moment he looks disappointed, but it turns to wonder as she softly plants her lips on his. As she pulls away, she tells him to have a good day. She really does want him to have a good day. It's probably that last good day he'll have for a long time. She leaves him while he's still bright red and stammering, and doesn't look back. She pushes him from her mind and goes in search of Black. Lestranges don't have friends. Lestranges don't love. They don't hold back for fear of hurting someone, and they certainly don't feel guilty.

Black, in his arrogance, is all too easy to lure into the forest. She almost wishes he'd been suspicious and unwilling to go with her, or that he'd put up a fight and people had seen and would know that it was her and not Newt that was responsible for this. But he laughs and winks at her, a conceited smirk spreading across his stunningly handsome face. "Knew you'd come around, Lestrange," he murmurs, his silk-smooth voice low in her ear as he slips his arm around her waist. She pulls him across the grounds, away from the crowds congregating at the quidditch pitch. Hufflepuff is playing Slytherin in a match that will decide the cup. The whole school is there. It's the perfect time to steal away quietly.

Black wants to linger on the fringes of the forest, but Leta giggles flirtatiously and asks if he's scared. Black laughs derisively and protests no more. His is a proud and noble family, pure-blood back to the beginning of time. There is nothing in this world that Bacillus Black has to fear. Or so he believes, Leta thinks viciously. Soon he'll see there are things he should be afraid of.

She can hear the footsteps of the beast. It can smell them. It's becoming agitated. She can hear its tail thrashing, its damaged wings flapping. They can see it now, and it can see them. A rush of heat blasts into Leta's face, even as the burst of flame from the beast's mouth hits Newt's wall of enchantment and spreads sideways instead of roasting them alive.

Bacillus Black is frozen to the spot. "Is that a – a?" He quivers, unable to get the word out. "Leta, we should go…"

"Isn't it glorious?" Leta marvels. The beast is magnificent. Fierce and furious and brimming with fire. An uncontrollable force of nature. Nothing can stop it. Inside the invisible barrier everything is burned to black emptiness.

"Leta, we need to go!" Black insists. The expression on his face is so terrified she can't help but let out a quiet giggle. She pulls out her wand and points it at the barrier.

Horrified realization replaces the terror on Black's face. "You're mad!" He exclaims, "Wait, stop! Don't let it out!"

Leta giggles again, watching a second burst of flame flash across the invisible wall. The heat ruffles her robes and hurts her eyes. She flicks her wand, replicating the pattern she'd seen Newt use when he let down the barrier to feed his creature. Out of the corner of her eye, she sees Black pull out his wand.

She spins to face him. "Expelliarmus!" She shrieks. Black's not ready for her. His wand goes flying and disappears amidst thousands of other sticks on the forest floor. He's never bothered to practice wandless magic and can't summon it back. He doesn't bother looking for it, just turns and runs.

"Impedimenta!" Leta screams. The spell hits him squarely in the back and he freezes. She advances on him. Behind her she can hear the creature throwing itself against the weakened barrier.

She stands over Black. "Say you're sorry," she snarls at him, wand pointed at the enclosure.

"I'm sorry," Black sobs, pale-faced and panicky, eyes never leaving the beast.

"Say it like you mean it."

"I'm sorry I laughed at you," Black says quickly, his smooth deep voice replaced by something high and breathy, quite unlike his own.

"And?"

"I'll never do it again?"

"Good," says Leta. She takes a second to savour the expression on his face Then she weakens the charm holding the beast in as far as she can (Newt has other safety measures in place, she's never seen him undo them all at once), and calmly makes her way across to the small wooden doll she'd left in an oak tree. The last thing she hears as it transports her away is Bacillus Black screaming for help as he clumsily dashes further into the forest.

The portkey deposits her in the room of requirement. She gets changes quickly and combs her hair. The dark waves have gone wild in all the excitement. There's a glow of triumph in her eyes. The person looking back at her in the mirror looks a little mad, but it's just the thrill of victory. Black will never think he's above her again. A moment of pure terror can bring even the most arrogant and entitled down to earth with a thump. Black knows what she's capable of now, and no-one can take her down. Even if Black chooses to humiliate himself by telling tales, there's no way he can prove it was her. She takes a moment to compose herself before going to join the rest of the school at the match.

The match is still in full swing when she gets there, 150-140 to Slytherin. Newt is sitting alone, his Hufflepuff scarf around his neck. For some reason he's ridiculously proud of being a Hufflepuff, even though the rest of his house are all a little ashamed of not being clever enough for Ravenclaw or brave enough for Gryffindor, or ambitious enough for Slytherin. Leta slides in next to him and kisses him on the cheek. Newt goes bright red again and won't meet her eyes, but he looks happy.

There's a sinking feeling in her stomach. The birds are taking flight in the forest. Soon that look will be gone from Newt's face. It's coming.

Leta tugs at his arm, drawing his attention away from the match. "We should go," she tells him. She thinks of how wide the fire spread, how searing hot it was even with the barrier holding back the flames. Suddenly she knows that Newt will get in the way. He'll try to distract it, to stop it, to protect the people from it, and it from the people. Newt is not going to get expelled. He's not going to go to prison. Newt is going to die.

She feels sick. "Come on, Newt, let's go." She knows now her plan was a bad one. She doesn't care if Bacillus Black lives or dies. She doesn't care about any of the teachers, or the classmates who laugh at her and call her names. But Newt – brave, clever, interesting Newt, her friend Newt, is going to disappear in a rush of blue-hot flame. She'll have no-one.

Newt looks into the sky over the Forbidden Forest and sees it. He glances down at her hand on his arm, hears her begging him to leave, and looks at her in horror. "Oh, Leta," he whispers, "Oh Leta, what have you done?"

There are tears rolling down her face, real ones. They feel different to the crocodile ones she's brought out for Newt in the past. They're colder and more painful. She wants to say she's sorry, but her throat has closed up and no words will come out.

Other students have seen it now – panic is spreading and people are rushing out of the stands, running in all directions. A large Gryffindor shoves his way between Leta and Newt and she loses him for a second. She sees him again, making his way to the centre of the pitch. "Stay still," he's telling people, "Be quiet, you'll frighten it." Nobody listens. Everyone's too afraid to listen to the mad boy whose two best friends are a giant squid and the daughter of a dark wizard. Leta clutches her knees to her chest and cries silently. She sits still and keeps quiet, like Newt is instructing.

It gets quieter once as many students as possible are safely inside the greenhouses, or sheltered by shield charms the teachers have put up. Newt is standing in the centre of the pitch, roaring. He's summoned his trunk from the boys' dormitory. It's lying open in front of him as his magically magnified voice lets out inhuman calls. He looks insane. Flames are spurting in spirals overhead. The air is full of smoke. One of the stands has caught on fire and is slowly disintegrating behind him. Leta watches through her hands, tears blurring her eyes.

Newt has his wand out as the creature circles lower. Leta gasps as a jet of flame bursts over him, but the flames disappear and Newt jumps aside unharmed. He must have frozen the fire, like witches used to do in the time of witch-burnings. These flames are fast and unpredictable, and no wizard can keep up with it for long, though. Newt circles his case, calling the beast. It follows him, swaying, circling lower. Then suddenly, Newt leaps inside his trunk, impossibly disappearing, and the creature dives in headfirst after him. The trunk snaps closed.

Leta waits, breath held. Waits and waits, until she's sure it's got him. He's dead. He must be. She wants to open the trunk to check, but that will let it out. So she sits there, hugging her knees, watching a piece of luggage sitting on the quidditch pitch. Around her, teachers are extinguishing fires and ushering students to their dormitories or the hospital wing. Leta doesn't move.

Everyone is gone but Leta and two of the teachers when Newt finally emerges, sooty and exhausted. He's limping slightly and has a large hole burned in the left side of his robes. The skin is red and blistered, but he's already put some kind of cream on himself and doesn't seem to be in too much pain. She wants to run to him, to throw her arms around him, to check he's really alive. She doesn't. He won't forgive her this time.

They magically lock the trunk and carry it between them as they escort Newt to the Headmaster's office. Leta can hear them talking of expulsion and endangering human life. She hears Newt admit to keeping it in the forest, begging for its life, saying it was hurt and needed help and he didn't know how it had broken the enchantments.

Leta feels sick. This is all her fault. Newt is losing everything because someone made a fool of her and she wanted to make them pay. A ball of guilt sits like lead in her stomach. She's sorry. She's so sorry.

But not sorry enough to own up.