In Harry Potter Sorc. Stone, Professor Quirinus Quirrell and Fluffy, the three-headed dog, are the threshold guardian. Fluffy is the first challenge the trio, Harry, Ron, and Hermione, has to pass through. Throughout the whole movie, Quirrell is suspicious and unknown as the villian's, Lord Voldemort's, sidekick until the very end. It was later found out that he was the one that let the troll out of the dungeon and the one that jinxed Harry's broom during Quidditch. Lord Voldemort took over part of Prof. Quirrell's body to basically survive. At the end, Lord Voldemort needed the Sorcerer's Stone and had Prof. Quirrell try to kill Harry to get it.