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The mountain troll is a breed of troll. They are the largest and the most vicious kind of their species. They are often depicted as bald and pale-grey. They sometimes use Graphorns as mounts, which results in scars due to the Graphorns not liking it.
A band of mountain trolls were wreaking havoc in Liechtenstein, which led to Pierre Bonaccord's position as Supreme Mugwump of the International Confederation of Wizards being challenged by the local wizarding community, as he tried to ban troll-hunting and grant them rights at the time.
The Troll that Professor Quirinus Quirrell let into Hogwarts School of Witchcraft and Wizardry during Halloween in 1991 was a mountain troll. It was defeated in the girl's bathroom by Harry Potter, Ron Weasley, and Hermione Granger. This troll was a diversion so that Professor Quirinus Quirrell could attempt to get past Fluffy to capture the Philosopher's Stone for his master, Lord Voldemort.
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