Slytherin

During his adulthood, in 993, Salazar Slytherin founded Hogwarts School of Witchcraft and Wizardry, with Godric Gryffindor, Helga Hufflepuff and Rowena Ravenclaw. He appeared to have carefully selected the members of his own House.

The qualities which Slytherin prized in his "handpicked students" included resourcefulness, determination, and a certain disregard for the rules, along with the ability to speak Parseltongue. He also selected his students according to their cunning, ambition, and blood purity.[12] His house within Hogwarts was symbolized by a serpent, with house colors of green and silver.

As a professor, he was in favour of teaching the Dark Arts. He sent one student on a task to study a Dark relic in the nearby Feldcroft Catacomb, which had the power to summon Inferi, which Slytherin recorded in his Spellbook. He also taught the Unforgivable Curses, such as the Imperius Curse, which were not illegal at the time.

He constructed a secret Scriptorium, which could only be accessed by those who spoke Parseltongue and had the willingness to cast the Cruciatus Curse on somebody else to enter. There, he kept his Spellbook, a tome that recorded his life and gave further insight into the Dark Arts. The secret of the Scriptorium passed through his descendants, the House of Gaunt, for generations, until it was eventually opened by Ominis Gaunt, along with Sebastian Sallow and their mutual friend, in the 1890–1891 school year.

Finally, and most famously, Salazar Slytherin was responsible for the construction of the Chamber of Secrets beneath the Dungeons of Hogwarts Castle. The Chamber contained a Basilisk which could only be controlled by his own descendant, the "Heir of Slytherin", to be used to purge the school of all Muggle-borns. In 1943, Tom Riddle, the Heir of Slytherin, opened the Chamber and used the Basilisk to attack Muggle-borns, resulting in the death of Myrtle Warren. It was opened again in 1992, by Ginny Weasley under the influence of Tom Riddle's Diary, which contained a fragment of his soul.

Source of information: https://harrypotter.fandom.com/wiki/Salazar_Slytherin