in the second book, "harry potter and the chamber of secrets", one of voldemort's horcruxes, his childhood diary possesses the body of a girl named ginny weasley and lets a monster called a basilisk into the school. the basilisk petrified several muggle-born students, turning them to stone. harry and his friends stop it, though harry's friend, hermione is also petrified in the process. she did left a clue behind, and harry destroyed the diary, restoring ginny's vitality.