The Whomping Willow is a very large, valuable, and uniquely violent species of magical tree located on the grounds of Hogwarts. Its branches function as limbs and will thrash and “whomp” at any perceived threat that approaches its trunk. The tree's violent behavior is its primary defense mechanism and a deliberate feature intended to guard a secret. The tree was planted on the Hogwarts grounds circa 1971, the same year that the werewolf Remus Lupin began his education at the school. Albus Dumbledore had the tree placed directly over the entrance to a secret tunnel leading from Hogwarts to the Shrieking Shack in the nearby village of Hogsmeade. This was done to provide Lupin with a secure and secluded location for his monthly transformations, protecting him and other students from harm. Each month, Madam Pomfrey would escort Lupin to the tree. Despite its violent nature, the Whomping Willow can be temporarily immobilized. A small, specific knot at the base of its trunk, when pressed with a long stick or branch, causes the entire tree to freeze, allowing safe passage into the tunnel beneath its roots.
The Whomping Willow plays a significant role in several key events throughout the series.