A History of Magic (by Bathilda Bagshot)

British Magical History: The First Wizarding School: Founders: Salazar Slytherin

Salazar Slytherin had many unusual magical talents. As a young boy, his friendship with Godric began when Salazar challenged Godric to a duel, then used Legilimency to anticipate the other boy's move and quickly disarm him. Too impressed to be offended, Godric offered friendship on the condition that Salazar never read his mind again. To even Salazar's own surprise, it was a promise that he kept for the entire time that they were on good terms.

Growing up, Salazar's wealthy parents kept him segregated from Muggles, never quite explaining to Salazar why they did so. One day, a Muggle boy was bullying a Muggle girl whom Salazar found quite beautiful. Seeing a serpent nearby, Salazar used Parseltongue to set the snake on the boy. The frightened girl ran back to her village to fetch adults, who returned to find the boy dead from the snake's venom. The girl implicated Salazar, and the boy's father went in search of the young wizard. When found, Salazar was nearly murdered in a brutal beating administered by the boy's father, but Salazar's own parents discovered the scene and killed the man.

This experience planted seeds of distrust and hatred in Salazar that later defined him. When he told Helga, Godric, and Rowena of his plan to ban Muggle-borns from Hogwarts, he was genuinely surprised at Godric's and Rowena's adamant refusals. Salazar had expected Helga to strongly disapprove, but hoped that with the support of the others he could convince her otherwise.

As Salazar had kept his promise to stay out of Godric's mind, he had no idea that his friend was Muggle-born. As for Rowena, she had never considered the idea that Muggle-borns were any different than pure-bloods, and when asked to consider the concept, she was able to quickly decide that she liked them both equally.

Full of bitterness and still hoping to 'cleanse' the school, Salazar created the Chamber of Secrets and placed a basilisk within that mirrored the hate in his heart. Godric, sensing that Salazar was practicing a very Dark magic, attempted to use Legilimency against him. Betrayed, Salazar pushed Godric out of his mind and with that contact finally discovered Godric's heritage. That push, along with the continued and ever-growing hostility of the other founders, convinced Salazar to leave Hogwarts, but not before preparing the Chamber.