The Mountain Troll is a minor antagonist in Harry Potter and the Philosopher's Stone. He was let into Hogwarts School of Witchcraft and Wizardry on Hallowe'en of 1991 by Quirinus Quirrell, possessed by Tom Marvolo Riddle, in an attempt to steal the Philosopher's Stone of Nicolas Flamel.
Quirinus Quirrell was able to control this troll, a rare talent. He said later that he had a gift with trolls. He also used a troll to help protect the Philosopher's Stone.
Role in the film
That evening, during Halloween, Harry notices Hermione's absence, and Neville tells him that Parvarti Patil said she's been drowning her sorrows in the girls' bathroom, making Ron feel guilt-ridden. At that moment, Professor Quirrell comes running into the Great Hall, shouting about a troll in the dungeon, then faints. All the students begin to panic until Dumbledore calms them down. He then instructs the Prefects to take the students back to the dormitories, and asks the teachers to accompany him to the dungeons.
As the Gryffindors head back to their common room, Harry and Ron suddenly remember Hermione and hurry off to warn her. Unfortunately, just as they arrive, the troll appears and enters the girls' bathroom. Inside, Hermione exits a stall, and immediately comes face-to-face with the troll, who proceeds to destroy the stalls with his club. Harry and Ron arrive just in time, and throw pieces of wood at the troll. Harry grabs hold of the troll's club and uses it to get onto the beast's shoulders, and shoves his wand up its nose, much to Ron's, and Hermione's, disgust. The troll furiously grabs Harry by the leg and swings its club at him. Ron, grabs his wand, and with a little guidance from Hermione, uses a levitation charm to make the troll's club levitate. The troll expresses surprise by its club's sudden disappearance, then looks up to see its club hovering above its head, before getting hit on the head by it. The troll drops Harry, stumbles, and topples to the floor, almost right on top of Harry. Harry prys his wand out of the troll's nose and cleans it off.
Professors McGonnagall, Quirrell, and Snape arrive; Professor McGonnagall orders Harry and Ron to explain what happened before Hermione steps in, claiming she went looking for the troll, because she read about them and thought she could take care of it, but she was wrong. As Professor McGonnagall berates Hermione for her stupidity, Harry notices a bite wound on Professor Snape's leg. She then gives Harry and Ron five points each for barely surviving against the troll. Professor Quirrell then asks Harry, Ron, and Hermione to leave the bathroom before the troll wakes up.