Timeline:
Prologue: Decades after Deathly Hallows.
Begins: Third Year, 1994. Chapter 21 - 'Hermione's Secret'.
Epilogue: the same timeframe as the prologue, but an altered-future.

Alterations to Canon:
Not explicitly, beyond fanon tropes.

Pairings:
Harry/Hermione.
Harry/Hermione/Susan/Padma/Tonks/Daphne/Luna/Pansy.
Harry/Harem/Multi.

Content:
Porn with a thin veneer of plot.

Possibly Dark-Harry and Dark-Hermione, but not Evil.
Potentially Dub-Con and Slaves.

Story:
During the tumult of the Dark Lord Voldemort's brief reign over Magical Britain and the Puppet Ministry's administration there were a lot of other crimes and activities apart from those carried out by Riddle and his followers. Many dark artefacts that were hidden away were freely used, many repressed and outlawed magics were used freely, eldritch abominations and demented creations of sorcery were loosed.

Amongst this chaos one particular collection of spellwork was looted from the Department of Mysteries and made its way to mainland Europe. A few years after the fall of Riddle a new threat rose up on the Continent, an incredibly powerful group of witches and wizards - the Coven - who only came into the open after they had already subverted significant parts of the governments, law-enforcement and business communities in Germany, France, Spain, Italy and several other magical governments on the Continent.

Their means of subjugation was also the source of their power and the further they spread their reach the stronger they grew. The spell books they'd acquired were on the subject of 'sex magic' and the Coven had used them to mentally enslave and bind thousands to their will and gathered power from every one of their victims.

Since the history of the world the supernatural and mystical aspects of magic and mythology have been repeatedly entwined with fertility and sexuality. This is no coincidence because sex is one of the most effective and efficient ways to create magic and can even be used to focus it and cast it.

No human skill, talent or ability even nearly approaches its potential to create life through reproduction. Wizards who divide the human race into magicals and muggles blinded themselves to the truth that all things which live have some measure of magic, in fact magic is the binding between the substance and materials that makes a living organism more than the sum of its constituent atoms. Reproduction is the act of combining the magic of two people to create a new source of magic. This was true even of muggles, despite their inability to take hold of, shape and channel their meagre magic consciously and for other purposes.

The nature of the spells that had been sealed away in the Department of Mysteries were in regards to how one could divert, capture and repurpose the magic unleashed by the act of sex, when two people's magical energies combined, intermingled and became more than the sum of their parts. Instead of being released to drive conception the magic could be drawn into the participants increasing their latent magical capacity (their 'cores'), a specific spell or ritual could be worked to retard the aging process, grant increased physical strength, resistance to damage, aid healing and cure sickness... or to bind the participants together.

Such bindings however didn't necessarily have to be mutually beneficial or equal in effect on both participants. The Coven had gradually plotted to seduce the most magically, politically and monetarily powerful individuals in these governments, increasing their magical power with each additional victim and simultaneously binding them as thralls, incapable of violating an explicit order given to them by their master.

Opposition forms but against such incredibly powerful witches and wizards with slaves they can order to do anything they need - slaves that they were continuously recruiting from their opposition's own ranks by compromising them through every weakness and vice - it was all terribly futile.

When the Coven comes to the United Kingdom the still largely inept and corrupt government falls swiftly to their campaign. Businessmen finding themselves bound by that prostitute or escort they hired, a politician discovers himself bound after a tryst with his mistress who was in fact a polyjuiced imposter, a Hit Wizard thinking he got lucky at the pub finds out he is anything but. Each subverted person provides an asset and access that paves the way to another target.

The resistance in Britain is completely underground and resort to magically binding all its members with something uncomfortably similar to a Dark Mark, ensuring identities can be confirmed and the brand kills any person if upon being bound by the Coven they are given an order that violates the magic laid down in the mark.

Harry executes a plan and the underground successfully capture one of the 12 leaders of the Coven. Under Veritaserum and torture they manage to extract information about the spells that the Coven uses, enough to reconstruct the magic-gathering, absorbing and enslavement components. But it is a great deal less than they had hoped to learn. The Coven members have compartmentalised the knowledge amongst themselves. Unable to get anything else out of their prisoner they execute him... and all across Europe tens of thousands of people die.

Every thrall of a Coven member is bound to die at the same time as their master.

They are faced with the reality that all those thralls who they hoped to free are doomed. Killing the entirety of the Coven would only kill hundreds of thousands of magicals. They have to find another way. In the meantime the witches of the Resistance conference and come to an agreement that the use of the sex magic spells will be vital to opposing the advance and spread of the Coven's influence. Partly on preference and partly because of his baseline magical strength the majority of the witches agree that Harry should become the Resistance's 'master'.

The use of the spells doesn't require absolute domination of will, like the Coven does, but in order for the magic to be drawn into one of the participants in the sex ritual then the magic of one participant must become submissive and subject to the other. The witches of the Resistance won't become slaves but they will become bound to Harry as a result.

Months of small and even large victories follow including when Harry, empowered with the magic gained through the sex rituals and spells, ambushes and subdues another Coven member, a witch, and then with the majority consent of the Resistance (mostly the women, the men becoming wary of Harry but nonetheless more fearful of the Coven or their brands) binds her to him but as a slave, in the fashion that the Coven does its victims.

With the incredible power of the enslaved Coven witch, who has acquired power from thousands of victims, Harry is now a serious threat to the Coven's interests and things become exceedingly more dangerous for the Resistance.

However the two most brilliant minds in the Resistance haven't been idle. Hermione and Padma have come up with a plan to undo the Coven's control. They started out trying to discover the nature of the Coven's spells by searching for anything and everything else that was looted from the Department of Mysteries and they've acquired research and materials looted from the Time Chamber.

They've constructed a very special ritual that should work to allow the minds of people to be transported back in time... but only two people in the Resistance qualify: Harry and Hermione. They are the only members who've time-travelled before, using a Time Turner in 1994. The ritual would start by 'pushing' Harry and Hermione back in time no further than a Time Turner could but then drastically increasing the amount of magic in the ritual so that it can 'tangle' this back-step with the one made in 1994 and push their minds and memories across all the intervening years and into the younger bodies.

The plan is only feasible now because they have a means of powering the prohibitively magic-expensive ritual, they have the sex magic spells. The ritual is prepared and Harry and Hermione working together have sex in turn with seven of witches. Six are the witches whose relationships with Harry have grown well beyond 'spell partners': - Susan, Padma, Tonks, Daphne, Luna and Pansy. The seventh is the Coven witch.

Before they begin the ritual Hermione gathers the memories of their six Resistance Witches and stores them inside her own Occlumency construct with the hope that in the past she will be able to return their memories. The gathered magic is funneled into the time ritual and Harry and Hermione are dispatched to their past.

Becoming the Villian:
Their plan seems to be complete with their successful time ritual, doesn't it? They've undone decades of events, the ritual and spell books that precipitated the crisis are back in the Department of Mysteries and with their future knowledge they can even ensure a better outcome to the imminent Blood War and defeat Voldemort early.

But the sex magic is intoxicating, enticing and seductive all of itself. Harry and Hermione use it between themselves to work rituals enhancing their own juvenile bodies, to increase their weak magic, enhance their sexual attributes, mature them physically and more. Harry and Hermione find their eyes wandering to the younger counterparts of the witches they knew in the future, in the Resistance.

Hermione and Harry, thinking of the importance of having more allies (and just coincidentally more participants for sex magic rituals), plan on how to restore the memories of Susan, Padma, Tonks, Daphne, Luna and Pansy. Though there are other ways to do so (or do almost anything) their mentality has become so accustomed to the effects of the sex magic that the methods and plans they conceive always resort to using sex magic to achieve almost all their goals.

Through the use of sex spells, lust potions, legilimency and other less than ethical methods they seduce each of the witches and use sex to impart the future memories of each witch to their younger selves. Harry and the seven witches only grow more ambitious as the subtle corruption of sex magic clouds their minds to moral qualms and dilemmas with the solution it offers by giving them magic to throw at everything.

Eventually their plan to break into the Department of Mysteries and dispose of the Sex Magic tomes become plans to acquire them and study them, of course to prepare spells that could break the control of any group like the Coven that might crop up in the future. Soon they are studying them for new ways to empower themselves.

The Seduction of Hogwarts:
It begins with Hermione and the other six witches working to gather more women to Harry. Though they aren't enslaved the rituals do make their magic submissive and subject to Harry and so their first instinct is to group themselves around him.

Dilys Derwent, the former Headmistress, and Victorian prude modified Hogwarts' wards to suppress sexual urges and amplify inhibitions, creating a ward that segregates the auras of witches and wizards from intermingling 'too greatly'. Ever since her tenure Hogwarts hasn't had more than a handful of pregnant students thanks to the spell and it has had unintended effects forcing most witches and wizards to experience interaction with the only the sexual energies in the auras of their own gender during their sexual maturation which has dramatically increased the rate of homosexuality and has also decreasing the birth rate in magical Europe as a whole.

Corrupted as they are Harry and Hermione are less interested in dismantling the ward as they are in repurposing it for their own needs. The segregation remains, blocking the interaction of the auras of witches and wizards, with the exception of Harry. The ward now lets only his aura intermingle with those of witches and then instead of dampening it amplifies Harry's and all witches auras, dramatically increasing the sexual urges of witches but only towards Harry and each other.

Apparently candid pictures of Harry naked in the Quidditch Showers start appearing in the girls dormitories all over Hogwarts, planted by Harry's girls. Sex toys are bespelled with charms similar to Fred and George's day-dream spells but about Harry, a little slow-release lust potion gets added to the shower-heads in the female bathrooms, small pamphlets with minor sex related spells are spread.

Gradually the eight of them become new Coven. Far more benevolent but no less power-hungry.

Epilogue:
The United Kingdom is basically Harry's fiefdom, run by hundreds of witches who are free but invariably submissive in their thoughts towards him and the other seven witches who are his Coven.