| “ | We'll teach just those whose ancestry's purest. | „ |
| ~ Slytherin's opinion regarding the running of Hogwarts. |
Salazar Slytherin is the posthumous overarching antagonist of the Harry Potter franchise. He is one of the four founders of Hogwarts who founded the House of Slytherin and the ancestor of Lord Voldemort.
Slytherin wanted Hogwarts to only accept pure blood students, when this proposal was rejected by all the other founders he became estranged and resorted to an extremist method to secure his vision for the school; murder alll Muggle-borns in Hogwarts to keep the blood purity from Pure-blood wizards intact. Slytherin's scheme to purge Hogwarts and his world views directly influenced all the actions of Lord Voldemort and his Death Eaters hundreds of years later.
History
Beginnings
Salazar Slytherin was one of the first recorded parselmouths (wizards that have the extremely rare ability to speak to snakes). When he founded Hogwarts with the other three founders: Godric Gryffindor, Helga Hufflepuff and Rowena Ravenclaw, he chose students who show traits of resourcefulness, leadership, ambition, cunning, shrewdness and determination.
He also had a friendly and harmonious relations with the other three founders, and all four coexisted peacefully.
Disillusionment, construction of the Chamber of Secrets, and departure from Hogwarts
Despite his harmonious coexistence and friendly relations with the other founders, Slytherin became disillusioned as time passed, wanting to be more selective of the students admitted to Hogwarts. He believed that the school should only accept pure blood witches and wizards, as he distrusted Muggle-borns, possibly of the great conflict between Wizardkind and Muggles. He tried to sway the other founders to agree with him but they refused, especially Godric Gryffindor. This caused Slytherin's friendship with Godric Gryffindor to break and resulted in him leaving the school. Before leaving, Slytherin built a secret hidden hall deep underneath the castle where he placed a Basilisk egg without the knowledge of the other founders.
Slytherin planned to have his "heir" eventually find that hidden room and unleash the Basilisk in order to kill all Muggle-born students in Hogwarts. This secret room came to be known as "The Chamber of Secrets" after the other founders heard rumours of its existence but were unable to find it.
Legacy
Future Hogwarts headmasters conducted multiple attempts to find Slytherin's Chamber of Secrets but all proved to be futile, leading the Chamber to be dismissed as a mere myth. As time passed and the castle was expanded, the Chamber was flooded by the addition of plumbing systems while a girl's bathroom was built over the original secret entrance, making the said bathroom the new entrance.
It is unknown when or how he died, but it is known that his portrait is still at Hogwarts and his locket was made into a horcrux by Voldemort.
Bloodline
Slytherin's bloodline was passed down through the female line for generations. The Gaunt family, who practised inbreeding by marrying their own cousins, in order to keep their bloodline pure and their ability to speak Parseltounge, were the last known family to possess Salazar Slytherin's blood. Among the last of the Gaunts included Marvolo Gaunt and his two children Morfin and Merope. Merope fell in love with a Muggle named Tom Riddle and eloped with him, while her father and brother were in Azkaban. Eventually Tom and Merope's marriage resulted in the birth of Tom Marvolo Riddle, later known as Lord Voldemort.
Riddle discovered that he was the heir of Slytherin during his time as a Hogwarts student, as such he found the Chamber of Secrets and initially followed Slytherin's plan. However, after the Basilisk killed Myrtle Warren, the school was threatened to be shut down. As such, Riddle was forced to stop accessing the Chamber and instead left one of his Horcruxes behind so that he could continue using the Basilisk through someone else in the future.
After Voldemort's death, everyone thought that Slytherin's bloodline had ended. More than 20 years after Voldemort's death, it's revealed that Voldemort had a daughter with Bellatrix Lestrange, her name was Delphini. As of 2020, Delphini is the last known survivor of the Slytherin bloodline.
