Slytherin house values ambition, cunning, leadership, and resourcefulness; the Sorting Hat said in Harry Potter and the Philosopher's Stone that Slytherins will do anything to get their way. The house mascot of Slytherin is the serpent, and the house colours are green and silver. Salazar Slytherin founded the house. The Head of House is Severus Snape until near the end of the sixth book. Then, Horace Slughorn, the previous Head of House, comes out of retirement re-assuming authority. The ghost of Slytherin house is The Bloody Baron. According to Rowling, Slytherin corresponds roughly to the element of water. The Slytherin dormitories and common room are reached through a bare stone wall in the dungeons. The Slytherin common room is a long, low, dungeon-style room, located under the Hogwarts Lake, furnished with green lamps and carved armchairs. The room is described in the second book as having a greenish glow.
The Sorting Hat claims that blood purity is a factor in selecting Slytherins, although this is not mentioned until the fifth book. There is no reason to believe, however, that Muggle-born students are not sorted there, merely that pure-blooded students are more desirable to that house, as there are several examples of half-bloods in the house (such as Snape and Voldemort). In Deathly Hallows, a group of Snatchers claim that "not many Mudbloods" are sorted into Slytherin.
When believing Harry to be dead and thinking that he has final victory in his grasp, Voldemort proclaims his intention to abolish the other three houses and force all Hogwarts students into Slytherin. This design is foiled by his defeat and death, after which Slytherin becomes more diluted in its blood purity, no longer remaining the pure-blood bastion it once was. Its dark reputation, however, does linger.