In the second volume of the series, a basilisk was the monster that inhabited the Chamber of Secrets. When student Tom Marvolo Riddle opened the Chamber of Secrets the Basilisk killed Moaning Myrtle, then hibernated for 50 years. During the events of the book, it was set loose again by a Horcrux of Lord Voldemort, and attempted to kill several Muggle-borns, but due to sheer luck all its victims were merely petrified. Spiders always flee from the Basilisk, as they are mortal enemies. The only thing the Basilisk seems to fear is the rooster, as the crowing of the rooster is fatal to a Basilisk. Tom Riddle's Horcrux commanded Ginny Weasley to kill all the school roosters, remarked upon by Hagrid. When Harry discovers the existence of the chamber and of its location, Riddle reveals his identity and sets the basilisk loose upon Harry while Ginny's life force ebbs away. Fawkes appears to assist Harry, blinding the basilisk with its talons and carrying the Sorting Hat; Harry pulls the sword of Godric Gryffindor from that hat, and uses it to impale the basilisk's head, killing it. The basilisk's fangs and its venom absorbed by the sword of Gryffindor proved instrumental for destroying most of Voldemort's Horcruxes.
In the Final volume of the series, after losing the sword of Gryffindor to Griphook, Ron and Hermione go to the Chamber of Secrets and pull some fangs out of the Basilisk's skull, and use one to destroy Helga Hufflepuff's cup.