Lord Voldemort
In the second book, Ginny Weasley comes into possession of a magic diary and is eventually possessed by Tom Riddle, a manifestation of Voldemort's younger self. Riddle possesses Ginny and makes her open the Chamber of Secrets, which allows a massive snake to terrorise the school and attack students who have non-magic parents. At the end of the book, Harry defeats the snake and destroys the diary, which also makes Tom Riddle's manifestation disappear.