Serpent of Slytherin

"....That is believed to be some sort of monster......"

- Professor Binns on the Chamber of Secrets occupant

Salazar Slytherin created the Chamber of Secrets at Hogwarts School of Witchcraft and Wizardry, and inside this Chamber was placed a Basilisk that could only be controlled by his one true heir (a Parselmouth).

Biography
It was born in the chamber, as revealed by Aragog. In the early 1940s, the Chamber of Secrets was opened by a Slytherin fifth-year, Tom Marvolo Riddle, who was able to control the creature through this method. Riddle unleashed the Basilisk on the school, his goal to kill the Muggle-born students. All the victims were merely Petrified, except for a student named Myrtle who was killed in the girls bathroom. This bathroom was also the entrance to the Chamber of Secrets. These events were nearly the cause for permanent closing of Hogwarts.

To avoid this fate for Hogwarts, Riddle blamed Gryffindor third-year Rubeus Hagrid for all the events, claiming the monster at fault was his pet Acromantula. Rubeus Hagrid was expelled, and the attacks stopped. However, Transfiguration teacher Albus Dumbledore became suspicious of Riddle and thus prevented him from further terrorizing the school from fear of discovery.

Slytherin's Basilisk would only listen to Tom Marvolo Riddle, regardless of Harry being able to understand and speak Parseltongue to it in the Chamber of Secrets. It is possible that the Basilisk was trained, enchanted, or persuaded to obey only the Heir of Slytherin, and not all Parselmouths.

During the battle between Harry Potter and the Basilisk, the Phoenix Fawkes was able to blind the Basilisk and buy Harry some time. During that period of hiding, Harry obtained the Godric Gryffindor's Sword and stabbed the Basilisk through the roof of its mouth, killing it. Harry subsequently used one of the creature's fangs to destroy Tom Riddle's Diary, which was being used to connect Voldemort's fragile soul to the material world.

Another one of the Basilisk's fangs was used again at a later time to destroy a similar object, Helga Hufflepuff's Cup.

Known Victims
The following list shows all known Basilisk victims both killed and petrified (and why those petrified weren't basically killed)

Killed

 * Moaning Myrtle
 * Two pieces of Tom Riddle's soul (in 1993 and 1998)

Petrified

 * Several unknown students during 1943
 * Mrs. Norris (saw its reflection in puddle of water)
 * Colin Creevey (saw through lens of camera)
 * Justin Finch-Fletchley (saw through Nearly-Headless Nick)
 * Nearly-Headless Nick (ghost already, and cannot die again)
 * Hermione Granger (saw in Penelope's mirror)
 * Penelope Clearwater (saw in her mirror)

Appearances

 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (game)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows