Whomping Willow on the Hogwarts grounds

This Whomping Willow was located on the the grounds of Hogwarts School of Witchcraft and Wizardry. It was planted around the year 1971, the same year that Remus Lupin began attending Hogwarts. It was placed to disguise the opening of a secret passage leading from the Hogwarts grounds to the Shrieking Shack in the village of 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. 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. The purpose of the tree 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.

History
In 1992, Harry Potter and Ron Weasley accidentally crashed the Weasleys' Flying Ford Anglia into the tree, seriously damaging it. The tree violently defended itself and nearly killed both boys. The following day, Herbology Professor Pomona Sprout had to bandage some of the tree's limbs to try and repair the damage, possibly assisted (or hindered) by then-Defence Against the Dark Arts Professor Gilderoy Lockhart.

In 1993, Harry Potter's Nimbus 2000 broomstick was destroyed when it fell from the air into the Whomping Willow's branches.

In 1994, Sirius Black, in his animagus form of a large black dog, dragged Ron Weasley into the secret passage beneath the Willow leading to Hogsmeade's Shrieking Shack. They were quickly followed by Harry and Hermione, then later by Lupin and Snape.

In 1998, Harry, Ron, and Hermione sneaked through the passage under the Willow to the Shrieking Shack during the Battle of Hogwarts. There, they witnessed Snape's murder by Lord Voldemort and his serpent, Nagini.