Overview
The departure of Salazar Slytherin from Hogwarts is one of the defining events of the school’s early history. What began as a philosophical disagreement between friends escalated into an irreparable rift that would cast a shadow over the wizarding world for a thousand years.
The Rift
For a time after the founding, the four founders worked together in harmony. Each sorted students into their own house according to their personal values, and the school flourished. Slytherin’s inclination toward the dark arts did not initially trouble the others---they were united in purpose and bound by deep friendship.
However, Slytherin’s anti-Muggle rhetoric grew increasingly strident. He believed that only pure-blood wizards---those born to magical families---were worthy of a magical education. While he already controlled which students entered his own house through the Sorting Hat, this was not enough for him. He demanded that all four houses refuse admission to anyone of Muggle birth.
The other founders rejected this demand without hesitation. Gryffindor, Ravenclaw, and Hufflepuff firmly believed that Muggle-born witches and wizards were every bit as valuable and deserving as their pure-blood peers. Even Godric Gryffindor, Slytherin’s oldest and closest friend, rebuked him for his intolerance.
Departure and the Chamber of Secrets
Slytherin left Hogwarts rather than accept a school open to Muggle-borns. But he did not leave it unblemished. Before departing, he secretly constructed the Chamber of Secrets deep beneath the castle---a hidden chamber containing a great stone statue in his own likeness and, more terribly, a giant venomous Basilisk. Slytherin’s hope was that one day a true heir of his bloodline would open the Chamber and unleash the serpent to rid the school of those he deemed unworthy.
Aftermath
Little is known of Slytherin’s life after his departure from Hogwarts. Many historians believe he died not long after leaving the school. His legacy, however, endured. The prejudice he championed---the belief in pure-blood supremacy---would poison the wizarding world for centuries.
His darkest ambition was realized roughly a thousand years later when Tom Riddle, a descendant of Slytherin, discovered and opened the Chamber. The serpent within was unleashed, and the cycle of terror Slytherin had set in motion finally came to fruition.