slytherin

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.