Unlock and purchase all forms of Lord Voldemort to unlock this trophy. There are 3 to collect, which if you are going for 100% you should get normally. You purchase these characters inside Madam Malkin's in Diagon Alley.
Professor Quirrell - The Face of the Enemy Level
Tom Riddle - The Basilisk Level
Lord Voldemort - Need 200 Gold Bricks, then go to the secret level in Borgin and Burkes