The Whomping Willow is used to conceal a secret path from Hogwarts ground, that leads to the inside of the Shreiking Shack.
The Whomping Willow was planted the same year that Remus Lupin (A Werewolf) came to Hogwarts as a young boy. It conceals the tunnel leading to the Shreiking Shack, which allowed Remus Lupin to transform into the magical creature during the full moon, without fearing for the other students safety. The Whomping Willow is first introduced in the second book, Harry Potter and the Chamber of Secrets, when Harry and Ron fly Mr. Weasleys car into the branches. The tree obviously retaliates at this strange object, attacking the car, causing the car to flee into the forest after ejecting the boys and their luggage.
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