Complex sentence: At Luna Lovegood's insistence, Harry speaks to Helena Ravenclaw's ghost, who reveals that Voldemort performed "dark magic" on her mother's diadem, which is in the Room of Requirement.
Split into simpler sentences: At Luna Lovegood's insistence, Harry speaks to Helena Ravenclaw's ghost. The ghost reveals that Voldemort performed "dark magic" on her mother's diadem, which is in the Room of Requirement.