The Whomping Willow

The Whomping Willow of the Hogwarts School of Witchcraft and Wizardry grounds is a very valuable, very violent tree which was planted in a solitary location on the school property. Whomping Willows attack anyone and anything that comes within range of its branches. There are two known ways to immobilize the particular Whomping Willow placed at Hogwarts: pressing a particular knot at the base of the tree or using the "Immobulus" spell. It was planted around the year 1971, the same year that Remus Lupin began attending Hogwarts. It was placed so to disguise the opening of a secret passage leading from Hogwarts to the Shrieking Shack in Hogsmeade. This allowed Lupin to travel unnoticed to and from the Shrieking Shack, where he was able to undergo his transformation as a werewolf in privacy. This was kept a secret from everyone but Hogwarts staff and Lupin's three closest friends, James Potter, Peter Pettigrew and Sirius Black. In time, Severus Snape also discovered its purpose.