Timeline:
Begins with Prologue decades after Deathly Hallows.
Time Travel to AU pre-Hogwarts, between 1981 and 1987.
Finishes, unknown.

Alterations to Canon:
Completely disregards canon epilogue.

Pairings:
Harry/Multi
Harry/Harem

Prologue:
Harry, wielder of all three Hallows, and long divorced from his brief time as Ginny Weasley's husband, is a drifter of sorts. With the ample vaults of the Potter and Black family at his disposal he travels the world, taking in the sights, learning new magic, meeting new people, going on adventures, occasionally doing something heroic and generally debauching himself with any willing women who turn his head.

His life of casual fun and sex comes to a rather abrupt end in 2034 when the Muggle world decides to lose its collective marbles and start lobbing around ICBMs, generally vapourising all civilization and sending the rest of the world into a nuclear winter. Harry dies from radiation poisoning several months after the bombs fall and finds himself drifting in a void.

Anthropomorphic Death appears before him and tells him that Harry cannot pass on to the afterlife, the Hallows provide immunity from being 'reaped'. If Harry wants he can be returned to the nuclear wasteland, restored, revitalised, rejuvenated and probably alone until radiation or some other death takes his body again, or... his soul can be sent on to be reborn in another universe, another time. Harry challenges the apparent helpfulness of Death, probing for hidden strings and traps in the offer and eventually secures the promise of being sent to whichever universe would be reflect his mixed desires for the family he never had, the adventure he craves, the challenge of magic and, of course, the satisfaction of his more base desires.

Setting:
The departure point of this world, where it differs from Harry's own, appears to have been during the final years of Gellert Grindelwald's campaign. The wizard had attempted to ascend to Godhood using an Ancient Egyptian ritual that he'd uncovered but had been thwarted at the final moment. What the magicals of Europe had initially dismissed as superstition and nonsense on the part of muggles suddenly reveals itself to be a threat to all life in Europe when they realise that the '10 plagues' of Biblical mythology are actually an account of an ancient Pharaoh's attempt to ascend.

The rivers of Europe turn blood red, actually contaminated with a small amount of Grindelwald's blood, and with little choice, most people in Europe, especially those in war ravaged areas, drink it, treated or not. This consumption of his blood binds them to the ritual and as each of the subsequent plagues that follow takes their turns ravaging the population of Europe, magical, muggle and animal, the life-force of each of the slain adds power to Grindelwald's ritual.

On the eve of the 10th and final plague, which would have struck down every remaining first-born male of every bloodline on the continent, Grindelwald's forces, his fortress and himself are assaulted and destroyed by the combined forces of Europe. They stop Grindelwald's ascension but that doesn't stop the ritual from collapsing, messily. The released, corrupted ritual energies flood across the world and creates an imbalance in the background magical energy of the world which will affect every witch and wizard who is born and grows up from then on.

In the first decade or two after his defeat the effects of 'Grindelwald's Curse', as it comes to be known, is seen in the drop in the incidence of male births. By the mid-1960s the ratio of female to male births is almost 7:1 and by the time Harry is born it is almost 12:1. Not only that but the average magical potential of wizards has been gradually but steadily declining since the ritual's backfire with more and more males born to magical families being either squibs or near-squibs.

This creates a lot of changes and strains on the social fabric of magical society across the world. Multiple marriage is the norm, almost any wizard who has the magic to finish his NEWTs has at least three wives, though it is not uncommon for a new witch to be added to his household every few years up until one of his wives successfully produces a magical male heir for his family line. In the Potter family it is Harry, born sixth of James' children, to his third wife Lily. James has four wives at the time Harry is conceived and only adds a fifth shortly before Harry's seventh birthday. Harry himself has ten sisters and one brother, who was just recently born.

Many old families find thanks to the inbreeding that proceeded the Curse's effects that they have low fertility and despite multiple wives they bear only a few children, all daughters, before infertility sets in. Thus many families are reduced to female heirs, each seeking to find a husband - preferably one who already has a son and thus a heir through another of his wives - hoping they can be the first of their sisters to bare a son who would be named the heir to their own family name.

With so much competition for a wizard a lot of witches get side-lined and find themselves with little hope for a marriage except to a muggle man, which carries its own stigma, not just for the witch but also for her children (wizards, rather than squib males, born of the union of a witch and a muggle are even rarer, less than 1 in 40). Muggleborn witches and such half-blood witches are usually bottom of the pecking order, last in line to get a wizard husband. Lily is a notable exception thanks to stunning beauty, forceful personality and accomplished magical skills - even then she was a third wife to James, who had two arranged marriages from a young age.

A lot of witches, desperate for a son and the power and security that will convey upon them in the magical world, actively seek to seduce married wizards, even with the stigma attached to infidelity and bastard children as the disparity in births has reached such a level that any male wizards born are important and valuable. Some such witches even force the wizard in question into marrying them to secure the son for his family, especially if it is his first son.

Resentment exists in many parts of this society. Wizards extracting substantial dowries from families looking to have their daughter's son take their family's name, rather than the father's. Skilled witches passed over not just for marriage but positions in business and government in favour of witches of better 'pedigree' and with better connections through their husbands. Squib and near-squib men who find themselves passed over by witches they desire who seek, either of their own ambition or their family's behest, a powerful wizard for a husband.

This is all further complicated by another effect of Grindelwald's Curse: a witch who remains a virgin, never having had sex with a wizard will eventually find her fertility disappearing, sometimes as early as her mid-20s, more often in their early-30s. This is true even of witches who marry muggle men. Even just losing her virginity to a wizard, having his magic enter her core, isn't enough to stave of infertility forever though, the longer a witch goes without sex with a wizard the more likely she is to become infertile. Some families seek to remedy this by encouraging consensual incestuous sex amongst siblings and cousins, carefully managing timing to avoid any pregnancies (since a contraceptive charm would negate benefits of the fertility magic altogether), others turn a blind eye to their daughters at Hogwarts losing their virginities to whomever they choose (even non-purebloods), preferably the strongest wizard they can persuade or seduce - hopefully leading to a marriage, though many unscrupulous wizards see the offered sex as their due and feel no obligations at all to the witches in question.

Amongst all this it should be noted that Tom Marvolo Riddle was born a squib and never even entered the magical world. There have been other dark magicals, more women than men, seeking to use forbidden magics (love potions being very, very illegal in this world due to even greater line theft concerns) to snare a husband, but nothing compared to the Blood Wars that Harry's world went through.

Story:
Harry's next memory is a hazy perception of Lily Potter shushing his cries of hunger and gently rocking him as a breast the size of his own head is provided for his attentions. He barely holds onto this awareness for a few minutes before the simple, contented happiness of satisfied hunger fills his infant body's mind and he loses awareness again. In quick succession the first six years of Harry's new life pass with him only occasionally conscious of what is happening before he slips away again, each time though he lingers in control for longer.

On his seventh birthday Harry wakes in complete control of his body and with total comprehension of what has passed in the previous years of his new life, all the knowledge of this new world he lives in. He carefully conceals his true nature, his intelligence and his skill but cultivates a reputation for being a prodigy and powerful. On his eleventh birthday he undergoes a series of rituals, the first measures his magical capacity, the second his fertility and sperm count, the third his heritage through his mother's bloodline.

The first ritual is rather straight forward, requiring the application of his blood to a magical artefact used in healing to determine the patient's magical strength in order to prepare an appropriate treatment for illnesses or curses. His result however is far from straight forward, he is already almost as powerful as his father, James, more than three times as powerful as any 11 year old ever recorded in the Potter family's history.

The second ritual involves Nymphadora Tonks, the best friend of Samantha, Harry's oldest sister, who has volunteered to carry the ritual out. She's been aware of young Harry's interest in her for some time and isn't averse to capitalising on it for a potential match with him when he turns 14 and the Potter family starts accepting betrothal negotiations. Her part in the ritual involves stimulating Harry - by whatever means necessary - until he ejaculates and collecting a sample of his semen for measurement. Harry's sperm count is likewise unprecedented (at least since Grindelwald's Curse) and a further test shows that a full half of the sperm are Y-chromosome carriers, a result so improbable that Harry is subjected (woe is him) to testing a second time, to confirm.

The third test, for any heritage through his mother, involves Gringotts. Many a muggleborn witch is driven to obtain a son, whatever the means, because only a son's magic, as a male heir of the line, can activate any dormant Gringotts vaults which have become inactive without a magical male member. It's the faint hope of many a muggleborn witch that she can have a life of comfort or even obtain a husband once she has thus demonstrated her heritage to a lost magical family. Harry's test reveals that Lily is the only descendent of two previously extinct families thought to have been wiped out during Grindelwald's plagues and the following effects of the Curse. This only serves to increase the influence and power the Potter family wields and that Harry will eventually possess.

Harry subsequently goes to Hogwarts and is soon surrounded by witches from 1st year to 7th year all hunting for a prospective husband. He is regularly propositioned and spends a lot of time entertaining and being entertained by various witches. His amazing skill with magic, talents at Quidditch and his ease with women even from a young age give him a notoriety and celebrity unmatched. It serves to create jealousy even amongst wizards who already possess good prospects and the attention of numerous witches. Draco in particular is even more antagonistic, because while his pedigree is good he is not a particularly powerful or fertile wizard. It is only exacerbated when Harry turns 14 and the Potter family is obliged by law to release their documentation of his magical potential and his fertility status. Harry's family is subsequently deluged with betrothal offers.

The Girls:
Hermione is befriended by Harry immediately and is, thanks to her upbringing outside the magical world, almost identical to the friend he remembers so fondly. If anything she's even more intelligent in this world and slightly more adventurous and outgoing. Harry adamantly refuses to entertain any witch displacing her from his side, regardless of whether she wants to be his friend or more in the future. Prejudice against Hermione is more for her closeness to Harry than her birth status, many seeing it as Lily Evans all over again, who had so ensnared James during their Hogwarts years. Eventually Hermione becomes his lover and even helps him choose his other betrothals.

Susan Bones is the oldest daughter of her family line and her betrothal to Harry has been inked and awaiting a signature (on his 14th birthday) since their were little, as friends who've known each other for a long time and get along well they are more than happy with their situation, though Susan has designs to secure a marriage to Harry for her best friend Hannah Abbott who is the daughter of the daughter of the last male Abbott and whose prospects for obtaining a husband are quite poor.

Daphne, the second oldest daughter of her family, is another certainty for betrothal upon his 14th birthday. She's extremely shy around Harry, brought out of her shell only by her friend Tracy who helps her be herself when around Harry and who loves Daphne as much more than a friend. Tracy's plan is to make herself integral to Harry and Daphne's relationship so she can stay with Daphne and also marry a man who her family will accept, even though she has an older brother and they would be more likely to try to get a good bride price for her from a family with a squib or near-squib heir desperate to get a wife (since they wouldn't need Tracy for line purposes).

Padma and Parvati who know the odds are against them, thanks to being a family of recent immigrants, cheat a little, using some grey-magic to scry out the most magically powerful wizard earlier than they would be able to find out legally and after Harry's magic overwhelmingly stands out against all but one or two seventh-years they make their play for him.

Veronica, this world's Ron, is more interested in girls at this stage and in fact develops a stunted, awkward crush on Hermione which brings her into Harry's orbit. Eventually she develops feelings for Harry, more friendly than romantic, but it's the relationships she starts with several of Harry's other witches that cements her place with him. (The Weasley family, from Molly: William, Charlotte, Persephone, Fred and George, Veronica and Ginevra. The also have five more half-sisters and one half-brother, through Arthur's other wives).

Cho, under orders to seduce the most powerful wizard she can get her hands on, by whatever means necessary. However she is held back by a  horrible childhood where she was molested cruelly by her father (never with actual sex, but making her 'learn' how to please a man) a near-squib wizard who cast aside Cho's mother, claiming she was at fault, when she became barren after Cho's birth. Terrified of receiving similar mistreatment from other wizards she only gradually finds it in herself to trust Harry's kind and friendly attentions.

Tonks becomes more and more certain over time that Harry's the wizard she wants, especially after she continues having fun with Harry during his first and her last year at Hogwarts. She and Harry's oldest sister Samantha are best friends, as good as sisters, though they enjoy each other's company in bed as well, and a marriage to Harry would make them family, despite the Black family's views on the Potters (her mother's marriage to a Muggleborn wizard, rarer than hen's teeth, isn't enough to see Andromeda cast out of the family in this world).

Fleur's life has been considerably even more harsh in this world, since Veela have an 'unnatural advantage' for luring men to them she is treated with suspicion and distrust at almost all times by witches afraid that she will poach their boyfriends, fiancées or even merely any potential male. Her father choosing to marry Appoline first rather than the more acceptable form of taking a Veela as a third or fourth wife, brings petty prejudice on him and his family. Her first encounter with Harry almost sees her bonding herself to him out of sheer reflex, the strength of his magic and his fertility hypnotic to her Veela senses.

Beside all these there are Harry's older half-sisters and numerous cousins who seek Harry out for companionship in bed, believing his stronger magic and fertility are their best hope to stay fertile and obtain a husband of their own.