Serpent of Slytherin

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

- Professor Binns on the Chamber of Secrets' occupant

The Serpent of Slytherin was a female basilisk placed by Salazar Slytherin inside the Chamber of Secrets at Hogwarts School of Witchcraft and Wizardry. The basilisk could only be controlled by Slytherin's one true heir, Tom Riddle. This means that despite her being a snake, other Parselmouths (such as Harry Potter) have no control over it.

Birth and Slumber
Salazar Slytherin's basilisk was born in the chamber, as revealed by Aragog. Slytherin, in his spite of the his fellow founders' acceptance of muggle-borns into the school, left a basilisk deep in the chamber, in hopes that one day his true heir will unleash her to purge all they deem unworthy to study magic. In order to prevent the serpent from dying from senescence, Slytherin placed her in suspended hibernation to prevent her from aging, until the heir would appear.

First Opening of the Chamber of Secrets
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 his status as the heir of Slytherin. Riddle unleashed the basilisk on the school, his goal being to rid the school of Muggle-born students. All the victims were merely Petrified, except for a student named Myrtle, who was killed in the girls' bathroom on the first floor. A sink in this bathroom concealed the entrance to the Chamber of Secrets. These events were nearly the cause for the permanent closing of Hogwarts.

To avoid this undesirable consequence for Hogwarts, which was the closest thing he had to a home, Riddle framed Gryffindor third year Rubeus Hagrid for all the events, claiming Hagrid's pet Acromantula Aragog was responsible for the attacks. Hagrid was expelled, and the attacks stopped. However, Transfiguration teacher Albus Dumbledore became suspicious of Riddle, and thus prevented him from further terrorising the school, since Riddle now feared discovery. Dumbledore also convinced Professor Armando Dippet, who was Headmaster at that time, to allow Hagrid to stay at Hogwarts as gamekeeper.

However, Riddle, not wanting his five years of hard work in finding out about the Chamber go to waste, left behind his childhood diary that contains a portion of his soul, in the hopes that one day it would reopen the chamber and reawaken the basilisk.

Second Opening of the Chamber of Secrets
Fifty years after Tom Riddle's original attack on Hogwarts, the basilisk was re-awakened by a shade of Riddle generated by a Horcrux he had housed in his old diary. This shade possessed Ginny Weasley after Lucius Malfoy slipped the diary into her cauldron during a confrontation in Flourish & Blotts, as part of his plan to discredit her father. However, although Riddle/Ginny killed the Hogwarts roosters to prevent them being used against the basilisk, the basilisk managed only to petrify a few students, a ghost, and a cat, each of them seeing only the snake's reflection or seeing it through something, rather than looking upon the snake directly. Using his own ability as a Parselmouth, Harry was able to hear the basilisk and attemped to find her. Later in their search Hermione became one of the basilisk's victims, but she had already managed to get some information on the basilisk for Harry and Ron, information which they found still clutched in her Petrified hand. Harry and Ron (accompanied by a reluctant Gilderoy Lockhart) entered the Chamber of Secrets, where Ginny Weasley was being held by Tom Riddle. After being separated from Ron and the self-Obliviated Lockhart, a battle commenced between Harry Potter and the basilisk.

Battle with Harry Potter and death
"Speak to me, Slytherin, greatest of the Hogwarts four!"

- Riddle's fragmented soul summoning the serpent.

During the battle between Harry Potter and the basilisk, Dumbledore's phoenix Fawkes was able to blind the basilisk, allowing Harry to fight the basilisk without the fear of being killed instantly by its gaze. 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, but one of its fangs pierced Harry in the sword arm as a result of the struggle, and splintered off. Harry subsequently used the said fang to destroy Tom Riddle's diary, which he would later discover was one of Voldemort's seven Horcruxes. Luckily, Fawkes' tears was used to save Harry from the otherwise fatal venom of the fang.

Post-mortem
By 1998 the basilisk's body had fully decomposed, leaving just a skeleton. Another one of the basilisk's fangs was used once again by Hermione Granger to destroy another Horcrux, Helga Hufflepuff's cup. The first fang's venom, which was absorbed by the sword that slew it due to its taking in only that which made it stronger, allowed it to destroy Slytherin's locket. While the sword slew Nagini, due to her being a living creature, it's unknown whether or not any sword would have done the job.

Traits
A horrifying monster with dark green scales, this creature was immensely large and had deep yellow eyes. It possessed great strength and was resistance to a majority of spells, and as a basilisk, it could kill with a gaze, and petrify with an indirect look. In addition, its fangs are so venomous, it can kill any living creatures it pierces within minutes, and were capable of destroying Horcruxes; the venom's potency remained even after five years of its death. Even when blinded (which took away its deadly gaze), it could still sense its prey through smell and sound.

It could breathe while submerged underwater, squeeze through the piping network of the school to go about undetected, and by entering a deep sleep, it ceases to age as long as remains asleep.

"Parseltongue won't save you now, Potter, it only obeys me!"

- Riddle's memory on the serpent's allegiance.

Slytherin's basilisk would only listen to the true heir of Slytherin, Tom Marvolo Riddle (also known as 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.

Known victims
The following list shows all known Basilisk victims both killed, Petrified, and otherwise harmed by the monster.

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 which is imbibed with a Basilisk's venom, destroyed Salazar Slytherin's Locket and Marvolo Gaunt's Ring, both of which contained a piece of Tom's soul.
 * The sword also slew Nagini, though the venom may not have been necessary to release the soul, due to the instability of living Horcruxes.
 * In the film version, a fang is also used to stab Rowena Ravenclaw's Diadem before it is engulfed by Fiendfyre.

Petrified

 * Several unknown students during 1943
 * Mrs. Norris (saw its reflection in puddle of water)
 * Colin Creevey (saw it through lens of camera)
 * Justin Finch-Fletchley (saw it through Nearly Headless Nick)
 * Nearly Headless Nick (made eye contact, but was a ghost, and could not die again)
 * Hermione Granger (saw it in Penelope's mirror)
 * Penelope Clearwater (saw it in her mirror)

Injuries

 * Harry Potter (her fang went into Harry's arm, who would have died if not healed by Fawkes)

Behind the scenes

 * In LEGO Harry Potter: Years 1-4, Harry does not get pierced by the Basilisk's fang and instead pulls it out of the dead Basilisk's mouth. Also, the basilisk notices Harry when Ginny screams at one of his disembodied eyes. A third difference is that the basilisk has a scarlet plume on its head, thus its gender is male. Additionally, Ginny helps Harry during the battle as opposed to being unconscious the whole time.
 * Ironically, the serpent that Voldemort plotted to use to purge the school of muggle-borns and Harry Potter was used against him by having its venom and fangs used to destroy most of his horcruxes, which led to his final downfall.
 * Penelope Clearwater is the only victim of the 1992–1993 school year incident who was not a victim of Slytherin's basilisk in the film adaptation of Harry Potter and the Chamber of Secrets. Still, there was a mirror next to Hermione's bed, but it is never mentioned that Penelope was with Hermione. Furthermore, in the film Cornelius Fudge claimed three muggles were attacked instead of four as in the novel.
 * In the film adaptation of Harry Potter and the Deathly Hallows, when Ron and Hermione enter the Chamber of Secrets, the skeleton of the basilisk is still there, but appears to be in a different spot than where Harry killed it earlier. Also, in the film, the fang was used to destroy an additional horcrux, Rowena Ravenclaw's Diadem.
 * In recalling his father reading the first two Harry Potter books to him, Daniel Radcliffe stated that his father had a great voice for the basilisk. He suggested his father to director Chris Columbus, and his father was mortified.
 * Various medias have shown both the male and female depictions of the species for Slytherin's basilisk.

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
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)
 * Harry Potter and the Deathly Hallows: Part 2
 * LEGO Harry Potter: Characters of the Magical World
 * LEGO Harry Potter: Years 1-4
 * LEGO Harry Potter: Years 5-7 
 * LEGO Creator: Harry Potter and the Chamber of Secrets
 * Harry Potter LEGO Sets
 * Harry Potter Trading Card Game
 * Pottermore
 * Harry Potter for Kinect

Notes and references
Basilisco di Salazar Serpeverde