Prologue: History of the Slytherin family
From two of the History books about the Magic in Great Britain and Magical family trees, owned by Hermione Jean Granger in 1994
Salazar Slytherin was the last Lord of famous Pure-blood House of Slytherin and later one of the four Founders of Hogwarts School of Witchcraft and Wizardry. Born in 962 A.D, he was said to be the result of a final attempt to have a male Heir, with his parents being almost too old to have children anymore. From an early age, he showed signs of having a very strong magic rarely seen in the Wizarding World at the time. Salazar was taught by the best and most respectable tutors in magic his parents could afford, and many were the Families who wanted to wed a daughter to the young Slytherin Heir once he had came of age.
In the 10th century, it was common practice in the Wizarding World to have the new generations, especially those of Pure-Blood families, marry early in life as magic, in all its glory and power, still was not a guarantee of survival to adulthood. As people normally entered adolescence very late by later time's standards, often in their past the age of fifteen, it became common to have the arranged marriages take place when both partners was seventeen years.
Marrying for love was seen as a mix of luxury and at the same time dangerous. Love was something that belonged inside a marriage, and having an unwed, lovestruck Heir or Heiress marry someone by following their own heart was often the start of a foolish event which could lead to a unhappy life once the first sparks of romance had faded.
In late summer 979 A.D, by the age of 17, Salazar Slytherin married his orphaned first-cousin Cearo who was just a few months younger than him. Both of them were a rarity in the Wizarding World by never having been betrothed to someone else before their marriage, but many later scholars would share the theory that it simply had been difficult to choose two good future spouses for Salazar and Cearo, and that it plainly seemed to be a better match between the two first-cousins as they were the only two Slytherin Heirs in their generation, both being an only child for different reasons, though there is old rumours that both Salazar and Cearo had been blessed with older siblings long before their respective birth, yet all those brothers and sisters had either been stillborn or died at a young age
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Lady Cearo Slytherin died of illness in 993 A.D. at the age of 31 years. During her 14 years long marriage to Lord Salazar Slytherin, her first-cousin by their fathers who were brothers, she is mentioned to have been with child twice. The first time, it was a stillbirth of unknown gender, yet most historical evidence suggests that it was a girl. The other birth, happening little over two years later, resulted in the birth of twins: Rinc, the heir to the Slytherin family, and his younger sister Sunniva. It is said that lady Cearo was harmed greatly in the birth, nearly costing her life to give her Lord and husband his children. She was never pregnant again because of her health becoming frail after the birth, leaving her only two children as legacy to history at her death when they were eleven years old. Lord Salazar Slytherin, being very close to his beloved cousin-wife all her life, was held in deep grief over her death and never remarried over the following seven years, even if history proved that it should have been a wise move to do so.
That same year, in September, he became one of the four Founders of Hogwarts and his 11-year-old twin children were two of the first students there. From the start, it soon were clear that the Founders had different views on which students that would be allowed to be schooled at Hogwarts. Yet despite that he was suspicious of Muggle-born students, Salazar managed to keep peace with his friends, if a problematic one thanks to differences in personality and his own fear that magic-fearing Muggles would find out about Hogwarts somehow.
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In the year 1000 A.D, the so called Great Tragedy of the Slytherin family happened: It all started with that Salazar's twin children Rinc and Sunniva was almost 18 years old this year and were set to marry a daughter from the House of Peverell and an Heir from the rather fresh House of Gaunt once all four youngsters had finished their schooling at Hogwarts. Now, the Peverell family was already known in the Pure-blood ranks, but the House of Gaunt had been created only two generations before, so for many of the families, it was a surprise that Salazar allowed his daughter to be betrothed to a such family. The reason of this choice have never been found.
However, Fate had different plans. The mystery behind lord Rinc's death is still unknown even today, but it is said that he and his father came into serious conflict with each other over a magical attack on a Muggle village at the Scottish border to England and that lord Salazar killed his only son in a sudden fit of rage, only coming to his senses when it was too late.
As mentioned above, Rinc died aged just seventeen years, and was the last officially known male Slytherin as Salazar himself never had remarried after the death of his wife. Nor had the young Heir of Slytherin gotten married and there is no evidence that neither father or son had illegitimate children to keep the family name remain in a male line. The lady Sunniva, Salazar's daughter, was hurried married only the very day before the duel as she and her eventual Gaunt husband had foolishly acted in a way that was only allowed for married couples at the time and made her pregnant before the marriage, forcing the two families to speed up the wedding before it became visible for everyone to see what the bride and groom had done.
No one really knows what happened after the fatal duel between father and son. Some says that Salazar went mad over his actions when he realized what had happened, killing himself in a desperate act of remorse and guilt over causing the death of his own Heir. Others say that he requested to be executed by Godric Gryffindor personally, for the crime of ending his direct male line of possible descendants, pleading himself guilty and viewing himself undeserved of pardon. His place of death and tomb is unknown, as he was not buried in his ancestral home together with his son and wife.
As the only remaining Slytherin family member left alive, Sunniva would not be spared from tragedy herself. No, her fate turned out to die in childbirth on 31 December 1000 A.D after giving birth to a single son, her cause of death rumored to be internal bleeding because of a difficult birth. Her son would survive to adulthood, and father a Pure-blood family name which would live on for the coming centuries.
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Author's note; The names of Salazar Slytherin's wife and children are actual Anglo-saxon names, which I felt was fitting since Hogwarts was founded in 993 A.D according to the HP wiki and it is mentioned in Chamber of Secrets that it is about a thousand years since it welcomed its first students. I also wanted to try explaining how the Gaunt family came to become his descendants later and why the Slytherin family name seem to be extinct in the male line