What happened in the whomping willow in the prisoner of Azkaban?

Answer 1

It was actually planted much earlier, to allow Sirius Black to go to school (as a path to the shrieking shack.)

It was actually planted to keep people from getting to the Shrieking Shack, as going under the tree was the only way to get there. It was meant to keep people from getting to Moony, aka Professor Lupin, who as we all know is a werewolf. It was to keep people from getting to him when he was at school and when he turned, as well as, to keep him from getting to other people. I think the third book tells you this.

Oh...and i almost forgot, I think it was actually enchanted during James', Sirius', and Lupin's years at Hogwarts.

Answer 2

Ron got dragged into its secret passage that Dumbledore created the year Professor Lupin got to Hogwarts by Sirius Black (in Animagi form. he was the large black dog Harry kept seeing, thinking it was the Grim). Harry and Hermione followed, found out is was Sirius, and Lupin came in too, along with Snape later, and they all (except Snape, who was knocked out cold with Harry's, Ron's, and Hermione's simultaneous Stunning spells) listened to Sirius and Lupin tell the story of the childhood and explain why Sirius was innocent.