In Harry Potter and the Chamber of Secrets, the Chamber’s monster is a Basilisk.
A Basilisk is a gigantic serpent associated with Salazar Slytherin, one of Hogwarts’ four founders. In the film, Tom Riddle’s memory explains that Slytherin created the Chamber and left behind a monster that could be controlled by his heir. The Basilisk is eventually released through Ginny Weasley, whose actions are manipulated by Riddle’s enchanted diary.
The creature moves through Hogwarts’ plumbing and attacks students while remaining mostly unseen. The victims survive because they do not look directly into its eyes: Mrs Norris sees a reflection, Colin Creevey views it through a camera, Justin Finch-Fletchley sees it through Nearly Headless Nick, and Hermione uses a mirror.
Harry confronts the Basilisk in the Chamber. Fawkes brings him the Sorting Hat, from which Harry draws Godric Gryffindor’s sword. He uses the sword to kill the serpent, although one fang wounds him before the battle ends.