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The Head of Slytherin House is a Professor at Hogwarts School of Witchcraft and Wizardry who is in charge of looking after and disciplining the students of their House. The Head of House also serves as a Professor of a subject. Most, if not all, of the heads of Slytherin house attended Hogwarts and were sorted into Slytherin. It is unknown how a professor would be elected for the post, but it's possible that the headmaster chooses the professor who was sorted into Slytherin and embodies the qualities that house founder Salazar Slytherin possessed such as determination, resourcefulness, and cunning.
Salazar Slytherin (c. 1000) Salazar Slytherin established the Slytherin House. He had a very strong belief that only pure-blood witches and wizards should be trained at Hogwarts School of Witchcraft and Wizardry. Because of his convictions, he often had disagreements with the other founders to the school, particularly Godric Gryffindor.
Horace Slughorn (between the 1920s and 1981) Horace was the Potions master and was well-known for his "Slug Club," a group of talented, well-connected, or famous students he favoured. Partly through the Slug Club, Horace developed a very close relationship with Tom Riddle. He told Tom about Horcruxes, believing the information he gave out would purely be used for educational purposes. When later incidents proved otherwise, Horace retired from his position, presumably because he felt ashamed of himself for the mistake he had made.
Severus Snape (between 1981 and 1990 - 1997) Severus was the Potions master at Hogwarts, and served as a Death Eater for awhile. When Snape learned of Voldemort's plan to kill the Potter family, likely including Lily Potter whom Severus had romantic feelings for, Snape served as a double agent, secretly working behind Voldemort's back for Dumbledore until his death. His career ended when Voldemort killed him in the hopes of gaining the full allegiance of the Elder Wand. Snape was a difficult teacher, and was disliked many students outside of Slytherin House. He had a special dislike for Harry Potter because of a grudge against Harry's father. It was also well-known that Snape deeply desired the Defence Against the Dark Arts position, but he never got his chance to teach that class until 1996.
Horace Slughorn (1997 - Pre 2016[1]) Horace, after being convinced by Dumbledore and Harry Potter, returned to Hogwarts as the Potions master. After Severus killed Dumbledore and became headmaster of Hogwarts, it was decided that Horace was to become the Head of Slytherin House once again. Following his second retirement, a portrait of Slughorn was placed in the common room.[1]
