Crossover:
No.

Timeline:
Fifth Year.

Alterations to Canon:
None before the story begins, besides fanon tropes probably.

Pairings:
Lavender/Harry/Hermione/Parvati.
Multi, possibly Harem.

Story:
Many generations ago, somewhere back in the Brown family tree, the Head or Heir of the line married a Succubus. Succubi, when well fed, are capable of appearing completely human and it is the dark secret of the supposedly pureblood family that at some point a Succubus entered their line. Every other generation or two a daughter of the family develops into a Succubus during puberty.

The family has managed to keep this 'contamination' secret for centuries, taking great pains to cover up the existence of the Succubi in their line, usually sending them away to join an enclave of Succubi living on the Continent. But in the lead up to Grindelwald's war prejudice against magical creatures like Veela, Succubi, Werewolves and other demi-humans saw the enclave go into hiding and they haven't been seen since.

Since then any Succubi born into the Brown family have been forced to receive a lump sum payoff from the family vaults in exchange for signing a contract that changes their name and binds them against ever speaking of their ancestry to anyone ever again before being turned loose to fend for themselves. Lavender witnessed it happen to an aunt of hers when she was just seven years old, a wonderful woman that she loved very much and then never saw again.

Lavender has held a deep seated fear that she might be a Succubus too, ever since she was old enough to understand what it meant. Ever since her first menstruation she has grown increasingly more lustful, being drawn to both boys and girls, receiving a special thrill when others admire her figure and looks, wet dreams are a constant and her reputation for being 'easy' isn't entirely unearned, usually being quite willing to trade a hand job for fingering in return after a couple of dates. She's never actually had penetrative sex before, however, fearing it might be something which would absolutely trigger a transformation.

She's taken extreme care to conceal these facts from her family, portraying a completely different version of herself to them when at home, which is why she comes into her inheritance while she is at Hogwarts instead of being kept home by suspicious family members. After a restless evening and unsettled night of sleep, she wakes up to find that her skin has become a pale violet colour, her eyes have turned golden and slitted, her hair has grown and her incisors are sharpened and lengthened.

Hermione and Parvati notice immediately, of course. Lavender swears them to silence and stammers out the truth to them. Parvati holds no prejudice, India's magical culture is replete with magical demi-humans though not as many of them are inter-fertile with humans as the species in Europe are. Hermione is likewise without judgement, having seen and experienced prejudice even while being entirely human, just because she is muggleborn.

Lavender says she needs to see Madam Pomfrey but there's no way she could hope to get to the Hospital Wing unnoticed and with Umbridge at Hogwarts it could be a threat to her very life, to the lives of her entire family if she were discovered. Parvati suggests maybe a glamour but Lavender says that they never stick for Succubi, part of their magic is their appearance and their form, the only way for a Succubus to appear human is to have sex with one regularly, to keep her hunger sated. Hermione's mind is racing and suddenly she remembers Harry's cloak and blurts out 'Harry can help'.

Lavender's thoughts immediately leap to the idea of Harry helping her by feeding her and asks, surprised, if Hermione is really okay with the idea of Harry helping her. Hermione, still not connecting the dots, waves it off. Harry wouldn't care that she is a Succubus, if Hermione asked she's sure he'd be happy to lend his invisibility cloak then Lavender could... the penny drops. Blushing and stammering Hermione insists that it's not like she's Harry's girlfriend or anything, if Lavender wants then, well, Harry can make up his own mind...

Lavender and Parvati scoff and roll their eyes. Parvati: 'As if you don't want to be, Hermione. I've hear you moaning his name in your sleep' and Lavender: 'Not yet, but come on Hermione, the two of you? It's obvious'. Mortified, Hermione escapes and goes to ask Harry for his cloak which he provides without question. A visit to Madam Pomfrey confirms what Lavender already knew, from her own extensive research, there is really nothing to be done for her, spell or potion-wise. Only feeding will give her the ability to pass herself off as human.

It's Parvati who asks if it needs to be a male who Lavender feeds from. A very uncomfortable and warm under the collar Madam Pomfrey confesses ignorance of the matter but Lavender explains that feeding off of a female would only sate her for a day, at most. A wizard on the other hand, the more powerful the better, could sate her for two or three weeks. Hermione, listening to all this talk of sex, her mind imagining Lavender and Parvati together or her with Harry, is turning her on fiercely. Back at Gryffindor Tower the girls are in their dorm when Lavender asks Hermione if she really would mind if Harry helped her.

Hermione is torn, why is Lavender asking her? At Lavender's deadpan stare, Hermione mumbles, asking why she couldn't chose someone else? Lavender shrugs, she doesn't trust any of the blokes she's ever fooled around with not to go running their mouths. Would Hermione trust anyone other than Harry if she was in Lavender's shoes? No, Hermione realised. With Umbridge having almost unlimited authority to do as she pleased it wouldn't be safe for Lavender. She and Lavender aren't the closest of friends but she would never want anything to happen to her.

Deciding Lavender's safety and, in fact, her health are more important Hermione goes and collects Harry, bringing him to an abandoned classroom where Lavender and Parvati are waiting. They explain the facts to him and an astonished and mildly panicked Harry agrees to help and is soon left alone in the room as Parvati and Hermione leave to ward the room silent and with a notice-me-not as well as standing guard under Harry's cloak - a very tight fit for the two girls who end up rubbing against each other quite a bit.

Harry is awkward at first but Lavender's hunger makes her quite assertive and soon they are getting down to business with Lavender being driven wild by her first time and Harry completing inside her several times. In the afterglow Lavender concentrates and, before Harry's eyes, reverts to her previous human appearance. She thanks Harry who stammers out that he was happy to help, Lavender smiling sinfully and saying that she doesn't doubt it.

Over the following days Harry and Hermione are awkward around each other, Hermione apparently not willing to make eye contact with him and maintaining a certain amount of physical distance. Harry is afraid she's angry at him, even though he only did what he did because Hermione herself asked him to. In reality Hermione can't stop imagining what went on in that classroom and rather than feeling jealousy she feels envy, she's 16 and she's never done more than kiss one boy and she's now more than aware that she's also bi-curious.

Hermione reassures Harry that nothing has changed, even though the words ring hollow. Two weeks later Lavender wakes up feeling the hunger again, even though she hasn't transformed, and soon Hermione and Parvati are on guard duty again as Harry services Lavender. This time though Hermione and Parvati, under the cloak, begin talking, speculating about what is going on inside and soon they are grinding against each other and start making out. Over the next week Hermione and Parvati continue their little fling, with Parvati being very assertive, driving Hermione wild and forcing her into confessing she wants watch Harry and Lavender, just as Parvati wants to.

Her spending more time with Parvati causes Harry to fear Hermione is pulling away from him and he takes her aside and apologises, for things with Lavender. Hermione tells him he has nothing to apologise for but Harry says he still feels like she's upset with him. Hermione assures him that she's not, that... that it's something else. She's unwilling to confess her rather perverted desires. Unable to find the words to convey his conflicted feelings Harry leans into her space, giving her the opportunity to back away and then kisses her when she doesn't.

Harry says that Lavender can find someone else, he's sure. Hermione's astonished, he'd really give up Lavender for her? Harry hugs her tight and says he'd give up anything to keep her with him, as a friend... or a lover, if she wants that. Hermione responds by snogging him silly. They spend an hour in a broom closet making out, fondling and grinding against each other. Afterwards Harry says he'll tell Lavender but Hermione tells him not to, that she's fine with it. Harry is incredulous and Hermione buries her face in his chest and mumbles that she's been experimenting with Parvati, that she likes girls too. She wants to keep having fun with Parvati, if that's okay with him. Not being a hypocrite and being the last to suggest two hot girls should stop making out, Harry assents.

Two weeks later and Lavender needs to feed again, this time thought Parvati and Hermione declare that they aren't leaving the room. They're staying to watch and in turn Lavender and Harry watch while Hermione and Parvati put on a show. Then Lavender and Parvati play while they watch Harry and Hermione and finally Parvati decides if it's good enough for both Hermione and Lavender she's up for it and has her turn.

It turns out that multiple partners at once means that Lavender can go much longer without needing to feed, with several weeks passing without her hunger returning. Despite this the urge, driven only by choice and affection, to be with her lovers sees her return to their embrace long before she even needs feeding again. At some point Harry starts joining them in their dorm at night, flying over to their window on his broom or Dobby transporting Harry across.

Epilogue:

Post-War.

The Potter family is hosting an annual victory celebration get together with friends and family, in this new world after the defeat of the Dark Lord and the tearing down of the prejudiced Ministry it doesn't even matter that Lavender is a Succubus and is 'out', wearing her true appearance among their many friends, with Hermione's, Parvati's and her own young children mingling with everyone else's children. Lavender's Succubus aunt is in attendance with her husband and wife lovers. Sirius is thrown into the pool by Amelia Bones after one too many over the top come-ons, Dean and Ginny along with Ron and Demelza are leading a game of pick-up Quidditch. Gabrielle is flirting with both Hermione and Harry, trying as she has at several of these anniversary parties, to convince them they needed to add another sexy demi-human to their family. Watching on as Hermione gives the young Veela a back-handed compliment which only causes Gabrielle to grin wider, makes Lavender suspect that the French woman's efforts are starting to work - and decides that she's not really opposed to the idea.