Whomping Willow

The Whomping Willow is a very valuable magical tree on the Hogwarts grounds. The tree is extremely violent, striking viciously with its branches at anything that comes within reach.

Headmaster Albus Dumbledore planted the Willow to guard a secret passageway to the Shrieking Shack, a building in Hogsmeade. Lupin was smuggled to the passageway each month at the full moon, where he could transform in the Shack into his werewolf form, without risking harm to others.