Hogwarts School of Witchcraft and Wizardry was founded around the year 990 A.D. by four of the greatest witches and wizards at the time: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin. They each represented an aspect of personality that they wanted to bring out in new students. There was just one problem: Slytherin had an argument with his co-founders about blood purity and wanted to admit only pure-blood wizards to the school. The other founders, especially Gryffindor, disagreed with him, causing him to leave, but not before secretly building the Chamber of Secrets and foretelling that his heir would open the chamber and unleash a murderous basilisk to purge the school of ALL muggle-born students.
In the year 1603, one of Slytherin's descendants, Isolt Sayre, was born. When she was just a girl, her kind parents were killed in a fire by her maternal aunt, Gormlaith Gaunt. For 12 years, she lived with her cruel aunt until she escaped to the New World. There, she would save and befriend a pukwudgie, whom she called William, after her late father, as he would refuse to tell her his true name. She also rescued and adopted two boys, Chadwick and Webster Boot, and rescued, fell in love with, and eventually married a No-Maj named James Steward. Unable to bring their adoptive sons to Hogwarts due to Gormlaith still searching for Isolt, the four decided to bring Hogwarts to them, and thus, in the year 1627, 637 years after Hogwarts was founded, Ilvermorny School of Witchcraft and Wizardry was founded, and thus, its legacy began. It was rumored, but never confirmed, that Isolt and James's biological daughter, Rionach Steward, was a Parselmouth. And so, to prevent Salazar Slytherin's power from being passed down any further, she chose to never marry. Her Squib sister, Martha Steward II, chose to marry a No-Maj of the Pocomtuc tribe.
Back in England, on December 31, 1926, a talented, yet sociopathic wizard was born: Tom Marvolo Riddle. During and after his time at Hogwarts, he split his soul into a total of eight fragments, which created a total of seven Horcruxes. He also abandoned his birth name and proclaimed himself Lord Voldemort.
July 31, 1980 was the day Harry Potter was born. When he was just one year and three months old, his parents were killed by Lord Voldemort. In a failed attempt to kill the infant Harry, Voldemort, not understanding the power of love, was ripped away from his own body, ending the First Wizarding War. Harry would go on to be known as the Boy Who Lived, as he was the only known survivor of the Killing Curse. On May 2, 1998, Harry would avenge his fallen friends and family and, thanks to he and his friends locating and destroying the horcruxes Voldemort created, destroy Voldemort once and for all, ending the Second Wizarding War. In the aftermath, at least 20 Death Eaters were killed and the rest (save for the Malfoy family, who defected at the last minute) were arrested, tried, convicted, and imprisoned.
Minister of Magic Kingsley Shacklebolt led a massive reform on the British Ministry of Magic to end the prejudice based on blood-status and to better combat Dark Wizards in the future. In addition, the International Confederation of Wizards created the Wizard's International Special Police force to help prevent future wizarding wars from happening.
With Martha's choice to marry and live the rest of her life as a N0-Maj in mind, her descendants, and thus Salazar's descendants from Isolt's side, would either be No-Majes/Muggles or No-Maj/Muggle-born wizards…