Timeline:
Begins in the Future, circa 2043.
Time-travel, back 49 years.
Continues in 1994, Third Year: Chapter 21 - 'Hermione's Secret'.

Alterations to Canon:
None beyond fanon tropes.

Pairings:
Harry/Multi.
Harry/Harem.

Content:
Potentially Dubious Consent, Dark/Grey Harry.

Prologue:
Being freed of the Horcrux after the Battle of Hogwarts drastically increases Harry's raw magical strength, no longer does his body need to split magical resources to hold the soul fragment in check, as well as a dramatic improvement in Harry's cognitive abilities, held back for most of his life by the close physical proximity of Tom's dark magic to Harry's brain.

He quickly becomes embittered by the swift return of the magical population to complacent indolence as a self-serving Ministry stymies progress, preferentially serves the interests of the rich and powerful and goes back to business as usual. Aware of how little restraint that such an unworthy system can effectively exercise against him, Harry begins to make waves in the magical world which quickly see him singled out and targeted once again by Ministry propaganda and paranoia.

Using his magic for obvious good gives many people a moderately favourable opinion of him or at least sees most hold no ill will. He helps law-enforcement both magical and muggle with anonymous tips, vital details, paper trails or recordings that help bust major criminal enterprises. He subtly, without violating the Statute, brings drought relief, blunts severe storms, restrains wildfires and facilitates rescues during disasters. But his bitterness doesn't let up as he watches the common folk fail to hold the powerful amongst them to account.

Slowly his thoughts turn gradually more Dark. He has no desire to bring pain or suffering on anyone but he comes to the firm belief that the vast majority of folks either lack the will, the wit or the desire to argue for what is rightfully theirs, defend what they supposedly value or adhere to the principles they supposedly believe in. All too willing to give away their portion of power, their vote, their say to the first persuasive crook or idiot who comes asking for it.

Though becoming a greater and greater pragmatism Harry never truly falls into the ends justifying the means but he has little sympathy for those who are plagued with troubles of their own making, nor is he in a hurry to save people from things they are perfectly capable of saving themselves from. With this shift of perception comes a refocusing of his efforts to help, delving into new magics, neglected magics, old and abandoned magics, frowned upon magics, outlawed magics and even daring to synthesis his efforts and explorations with muggle knowledge and scientific method.

Walking this path leads him to many discoveries but it is two which constitute the greatest of importance. Firstly, fertility magic is revealed to be the most effective means of obtaining a new, abundant source of magic to power spells or even add to a wizards capacity or reserve. The potential procreative power of sex, the ability to make life, can be harnessed and re-purposed. Secondly, that time-travel beyond a few hours into the past is indeed possible, with the proper calculations, the necessary preparations and in an odd twist, for someone who has already time-travelled before.

Forty-nine years are unwound by Harry's ritual, entangling one time-turner's loop with the one around his and Hermione's necks in 1994 the night they went back to save Sirius.

Story:
- Now returned to his past Harry has the whole future and all his foreknowledge at his disposal. He immediately begins working to stack the deck in his favour, ready to take advantage of situations and opportunities he knew about in the future, but with the benefit of hindsight.

- During the summer between Third and Fourth Year he works hard, potions to reverse stunted growth thanks to under-nourishment, acquiring lawyers for Sirius and Harry's soon to be business interests, fake identities to start holding companies in the muggle world, memory charming muggles to buy winning lottery tickets and pay a decent portion of their winnings into his accounts, etc.

- Filing a petition for emancipation in the knowledge that being told he has to compete in the Triwizard Tournament, a competition restricted to of-age competitors as determined by the British Ministry of Magic, will almost certainly validate his claim - but bribes being ready as needed, just in case.

- After Harry becomes emancipated he files claims on the estates and accounts of any Ancient, Noble or Ancient and Noble houses whose last Lord had a will containing an 'Inheritance by Retribution' clause. Becoming popular when Grindelwald was scourging Europe such clauses would give the family assets, monies, estates and lineage to anyone who brought death upon the person who killed the last of their family if that person also agrees to restore the line by marriage and blood-adopting the resulting children of that union with blood provided by the last Lord of the family, kept preserved in the family's vault. It simultaneously served as a means to resurrect their family if it was destroyed by a hostile enemy and provided incentive to ambitious persons to avenge their family's death with the promise of a substantial reward. Needless to say many families that were exterminated under Voldemort's first reign of terror possessed such clauses from decades earlier and so Harry becomes the (potential) head of many families - each requiring a separate wife. This doesn't even take into consideration his own claim to the Potters, possible claim on Peverells or his intention to take, by conquest, the Lines of Gaunt and Slytherin.

- Harry ensures that his sudden extreme eligibility for marriage is well publicised and is soon receiving a variety of enticing proposals, all while carefully cultivating a lot of promising opportunities amongst the Hogwarts' female student body.

- Candid naked pictures of Harry in the Quidditch locker room start turning up, Harry wears a cologne derived from Amortentia and tweaked to make him smell like whatever any given witch finds the most appealing or attractive, a Dream Casting ritual performed on the nights of a new moon allows Harry to reach out through the aether and assert a little control over any dream any person in the castle is having which features him and makes said dreams all the move vivid and stimulating.

- Harry readily and capably makes excellent use of the sex offered by a great many of Hogwarts' witches to power rituals and spells that enhance his magical strength, empower his body, enchant artefacts and many more things besides.

- Before the end of the Triwizard Tournament he has hunted down all of Tom's horcruxes and places them in a prepared ritual space, ready to be cleansed/destroyed the moment Harry triggers it. In the Graveyard, covertly prepared in advance, Harry allows Voldemort to regain his body and summon his Death Eaters before triggering the broadcast of what is occuring to a magical holographic projection system at Hogwarts so that everyone can witness as he absolutely mops the floor with Tom, unmasks and dispatches his Death Eaters and finally ends Tom for good.

- With that taken care of Harry has utterly cemented his power and influence over Magical Britain. It will only be a few years until Harry has seized control over all the truly vital parts of government, his hands on the strings and levers, able to control the course of the Ministry and the views of the public essentially at will. A sort of 'Grey Lord' far more proactive and interventionist than Dumbledore but willing to let the politicians practice their Machiavellian arts against the targets of his choice, and for the public masses of sheep to graze as they please, though only on prepared opinions of Harry's own choosing.

- Revenge and Bashing done with subtlety and preferably while directing suspicion and ire for his victims towards other enemies or opponents of Harry's.

- Pregnant witches have their fertility magic diverted to nurturing their offspring and cannot be participants in fertility magic rituals, Harry can avoid impregnating witches he marries to satisfy the lines he inherited for only so long - at least the already of-age ones, he can hold of regarding witches who attend Hogwarts with him, at least until graduation. This means that while still attending Hogwarts he has wives who may be seeking conjugal visits during the school year.

- Perhaps some of the lesser known Professors, like Babbling, Sinestra or Vector, should try to seduce Harry to potentially become the ladies of important Lines, seeing an opportunity to move up in social status when traditionally becoming an educator has been a role for witches whose intellectual pursuits distracted or diminished their chance to 'marry up'. This is a situation Harry should be entirely pleased to take advantage of.