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, Tom Riddle. This being said, even other Parselmouths, such as Harry Potter, have no control over it.

First Opening of the Chamber of Secrets
Salazar Sytherin's Basilisk 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 girl's bathroom. A sink in this bathroom was 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 Aragog, Hagrid's pet Acromantula. 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. He also convinced Professor Armando Dippet, who was Headmaster at that time, to allow Hagrid to stay at Hogwarts as gamekeeper.

Second Opening of the Chamber of Secrets
Slytherin's Basilisk would only listen to Tom Marvolo Riddle (later Lord Voldemort), regardless of Harry Potter 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. She was found again by Tom Riddle's memory, who ordered her to kill every Muggle-born student. However, the Basilisk managed only to petrify some students, a ghost, and a cat.

Harry Potter, Ron Weasley and Hermione Granger learned about the Basilisk and attemped to find her. However, Hermione was one of the Basilisk's victims, but she managed to get some information on the Basilisk for Harry and Ron. Harry and Ron went to the Chamber of Secrets, where Ginny Weasley was held by Tom Riddle. A battle started between Harry Potter and the Basilisk.

Battle with Harry Potter and death
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 Godric Gryffindor's Sword from the Sorting Hat, which had been brought to him by Fawkes. Harry 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 one of Voldemort's seven Horcruxes.

Postmortem
Another one of the Basilisk's fangs was used again at a later time to destroy another Horcrux, Helga Hufflepuff's Cup. The venomous properties of the Basilisk absorbed by the Sword that slew it allowed it to destroy Slytherin's locket. While the sword slew Nagini, due to being a living creature it's unknown whether or not any sword would have done the job.

Known Victims
The following list shows all known Basilisk victims both killed, petrified, and others she harmed.

Killed

 * Moaning Myrtle
 * Postmortem: fangs and venom destroyed two pieces of Tom Riddle's soul (in Tom Riddle's Diary in 1993 and Helga Hufflepuff's Cup in 1998.)
 * Indirectly: Godric Gryffindor's Sword absorbed this Basilisk's venom and destroyed Salazar Slytherin's Locket and Marvolo Gaunt's Ring, both of which contained a piece of Tom's soul.
 * The sword also slayed Nagini though the venom absorption may not have been necessary to release the soul due to the instability of living Horcruxes.

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)

Injuries

 * Harry Potter (her fang went into Harry's arm)

Appearances

 * 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 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 (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows
 * Lego Harry Potter: Years 1-4