I would say the most important event so far is Fiver's prediction about the Sandelford Warren's destruction. Fiver is this little rabbit who is really weak, but is kind of psychic. He has a vision that the warren he lives in is going to be destroyed by humans. He and his brother Hazel round up 9 other rabbits and leave to find a new home, which is the plot for the first part of the story. This event is pretty much what started the whole plot.