Parseltongue

Parseltongue is the language of snakes and those who can converse with them. Those who can speak Parseltongue are known as Parselmouths. It is a very uncommon skill, and may be hereditary. Nearly all known Parselmouths are descended from Salazar Slytherin.

The ability to speak Parseltongue is considered an attribute of a Dark Wizard, which is partly due to the fact that both Salazar Slytherin and Lord Voldemort have this ability.

Harry Potter as a Parselmouth
Voldemort passed the ability to speak and understand Parseltongue on to Harry Potter when he attacked him in infancy, though Harry was unaware of this until 1992; he was pitted against Draco Malfoy in the Dueling Club and was able to communicate with the snake Draco conjured to attack him. This caused hysteria among his fellow students because graffiti on the walls of Hogwarts foretold that the Heir of Slytherin would open the Chamber of Secrets, unleashing a monster that would attack the school's Muggle-born students. The Heir of Slytherin was in fact Voldemort, but Harry was able to gain access to the Chamber by speaking the password in Parseltongue, and subsequently killed the basilisk within.

Harry also used his skill in Parseltongue to open Salazar Slytherin's locket. When Harry, Ron Weasley, and Hermione Granger returned to Hogwarts to search for Voldemort's final Horcrux, Ron managed a weak imitation of Parseltongue to re-open the Chamber of Secrets so that he and Hermione could fetch basilisk fangs, which Hermione used to destroy Helga Hufflepuff's Cup.

After Lord Voldemort destroyed the fragment of his soul residing in Harry, Harry lost the ability to speak Parseltongue, which he was glad for.

Known Parselmouths

 * Marvolo Gaunt
 * Merope Gaunt
 * Morfin Gaunt
 * Herpo the Foul
 * Harry Potter (from 1981 to 1998)
 * Tom Marvolo Riddle
 * Salazar Slytherin
 * Mrs. Gaunt (possibly)

Etymology
J.K. Rowling has said that she took the name Parselmouth from an "old word for someone who has a problem with the mouth, like a hare lip.".

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Deathly Hallows