Disclaimer: I do not own Harry Potter or any affiliated works, beyond the copies of the books and movies that I have purchased to read. This is purely a fanfiction that has been written for my enjoyment and I am not making any money off of this.
Chapter One: How Things Change By Adding One Witch
Most witches and wizards the world over think that they know the story of one Harry Potter and the "Heir of Slytherin," Lord Voldemort. What none of them realize is that they have only about half the story. No one knew that Voldemort wasn't the Heir of Slytherin, for Salazar Slytherin had no children though he was the eldest of 4 siblings. No one knew that it was the youngest sibling, one Guinevere Elizabeth Marie Salazar Isabella Rose Slytherin, that he had named his Heiress. And no one knew that the youngest sibling, his beloved sister, was still alive, or that she had befriended Harry Potter alongside Hermione Granger and Ron Weasley. In fact, Guinevere and Hermione had a friendly rivalry over who could get the best grades in their classes.
His favorite sister was still alive because Magic itself dictated that she had to live. As a result of this decision, had sent one of Their Priestesses to ensure that Guinevere would live in a dimensional pocket unable to age until certain events have come to pass, upon which she would return to her original dimension. The existence of the Priests and Priestesses of magic would be forgotten in the annuals of history within the next century as the mundane churches of the 10th century took their prosecution of those with magic to a higher degree.
Which leads me into the next set of facts no one knows in the story of Harry Potter and Voldemort. No one knew that of all the founders Salazar Slytherin had suffered the most because of mundanes. Mundanes had killed his parents and his younger brother Tonrac. A mundane man had forced himself on his other sister Carra who committed suicide soon after. If Tonrac hadn't already been married and a father then the line of descendants that Voldemort held claim to wouldn't have existed. No one knew that the only reason Salazar didn't approve of mundane-born, aka muggleborns, attending Hogwarts was because he could not stand the idea of losing any of their students. After all, 9 times out of 10, when an actual magical was caught and killed or raped by a mundane it was a mundane-born magical.
Most witches and wizards hear of the argument on the subject between the founders and classify Salazar evil and dark, without taking into consideration the fact that back when Salazar was alive mundanes were well known to hunt and kill magical and anyone that was conceivably accused of practicing witchcraft or devil worship. Most witches and wizards have come to view mundanes as harmless to them as if the very idea of a mundane actually being able to harm a witch or wizard as absolutely preposterous and impossible forgetting that it wasn't until the founding of Hogwarts that knowledge of several vital spells such as the Flame-Freezing Charm became as widespread as they are in the modern-day era.
Most people think that one's magical education has always started at age eleven, and again they would be wrong. Prior to the founding of Hogwarts, the education of a magical would begin as soon as it was possible to do so, and even then the minimum age to 'legally' learn magic wasn't a thing until the International Statute of Secrecy was passed. For mundane-borns that meant as soon as an older, more experienced magical finds them. For those born into magical families that meant they would start to learn magic almost as soon as one first displays a case of accidental magic. This usually made it easier for one to hide from mundanes as it meant those magicals had better control of there magic. It also ensured that family magic was passed on. And, if there was one thing Guinevere knew that she could pride herself on it was her hard-earned mastery of her family magic, aka the ability to use Parselmagic and several different kinds of elemental magic that have since been long forgotten, and the magic that would equate to the first year and a half of a formal magical education. A mastery that she had attained before even turning 11.
The last fact that most people weren't aware of has no actual bearings to their daily lives even if it is still important to the story of Harry Potter and Lord Voldemort. It was with the passing of the International Statute of Secrecy that the Restriction of Underage Magic was developed and also passed and with it the creation of the Trace. The Trace that detects underage magic was initially a worldwide spell that was cast around the same time as the International Statue of Secrecy, and during the initial casting, they had to manually add any-known underage Magical for it to take action. As such, Guinevere who was long since forgotten and hidden away by the Priests and Priestesses of Magic as they had been directed wasn't included. Later on, after meeting Harry, Ron, and Hermione she made use of some of the knowledge bestowed upon her by Magic itself to erase Harry Potter and Hermione Granger from the effects of the Trace by the end of First Year without letting anyone but them know, warning them not to mention anything. She also took it upon herself to teach the trio the basics of wandless magic as she, herself had been taught by her brothers.
It was all of these unknown facts and the differences caused by the actions of Guinevere and the reactions of Harry, Ron, and Hermione that caused the changes in how the story of Harry Potter and Lord Voldemort occurred.