Slytherin is one of the four Houses of Hogwarts at Hogwarts School of Witchcraft and Wizardry. Students sorted into Slytherin are known for their ambition, leadership qualities, cunning, and resourcefulness. The House has a strong emphasis on blood purity, a legacy of its founder, Salazar Slytherin, who believed that magical education should be reserved for those of pure-blood ancestry. Due to this legacy and its production of numerous infamous Dark Wizards, including Lord Voldemort, Slytherin House has a dark and often maligned reputation. Many in the wizarding world associate the house with the Dark Arts and untrustworthy behaviour. This rivalry is most pronounced with Gryffindor House, which was founded by Salazar Slytherin's primary ideological opponent, Godric Gryffindor. However, not all Slytherins fit this stereotype. Characters like Horace Slughorn fought against Lord Voldemort, and Regulus Black sacrificed himself in an attempt to destroy one of Voldemort's Horcruxes. Andromeda Tonks, née Black, was disowned by her family for marrying a Muggle-born wizard. Severus Snape, despite his complex allegiance and difficult personality, ultimately played a crucial role as a double agent for the Order of the Phoenix.
Slytherin House was founded by Salazar Slytherin, one of the four founders of Hogwarts. His insistence on admitting only pure-blood students led to a severe disagreement with the other founders, particularly Godric Gryffindor. The conflict culminated in Slytherin leaving the school. Before his departure, Slytherin secretly constructed the Chamber of Secrets deep within the castle. He sealed a Basilisk inside, a serpent only his heir could control, with the intent that it would one day be used to purge the school of all those Slytherin deemed unworthy to study magic—namely, Muggle-born students. The Chamber was opened twice in the 20th century: once by Tom Riddle in the 1940s, resulting in the death of Myrtle Warren, and again in 1992 by the memory of Tom Riddle preserved in his diary.
The entrance to the Slytherin common room is located in the dungeons of Hogwarts Castle. It is concealed behind a stretch of bare, damp stone wall. A password is required to make a stone door slide open. The common room itself is a long, low, dungeon-like room situated underneath the Great Lake. It is decorated with rough stone walls and a ceiling from which greenish, globe-shaped lamps hang on chains. The room has a grand, elaborately carved fireplace and is furnished with low-backed, black and dark green leather sofas and chairs. The windows of the common room look out into the depths of the lake, giving the entire space a greenish, underwater glow.
Slytherin House serves as the primary antagonist house to Harry Potter and his friends in Gryffindor. Harry's school rival, Draco Malfoy, is a prominent Slytherin, as are his cronies, Vincent Crabbe and Gregory Goyle. The house is intrinsically linked to the main conflict of the series, as Lord Voldemort himself was the Heir of Slytherin. Throughout the books, the Slytherin Quidditch team is a major rival to the Gryffindor Quidditch team, often engaging in foul play. In the Second Wizarding War, many Slytherin students and their families aligned themselves with the Death Eaters. During the Battle of Hogwarts, the Slytherins were initially led from the Great Hall to avoid conflict. However, a significant number, led by Horace Slughorn, later returned with reinforcements from Hogsmeade to fight against Voldemort and his army, demonstrating that the house's allegiance was not monolithic. In the epilogue, Harry's son, Albus Potter, expresses fear of being sorted into Slytherin, but Harry reassures him that it would be fine, noting that one of the bravest men he ever knew, Severus Snape, was a Slytherin.