It is a monstrous serpent, said to be the king of all snakes. It is born from a rooster's egg incubated under a toad and has a deadly gaze that can kill instantly.
Basilisk was in the Chamber of Secrets, a hidden chamber within Hogwarts School of Witchcraft and Wizardry. It was released by Tom Riddle, the young Lord Voldemort, to terrorize the school and eliminate Muggle-born students.