I know it's not much to go on, as I read this book probably close to 7-9 years ago. Here's my best synopsis of what I remember. It is also part of a series, if that helps.
There is a magical tree that the people of the land kind of worship, kind of serve. It hasn't spoken in many many years, but one day it does. It speaks to one of the monks/disciples that care for it. (From what I remember, this is a very important and selective job). It tells the person that it is dying and they need to plant a new seed (I think???). The main part that I remember is that this tree was the force protecting their world from evil. Many years ago there was a great war and all the evil was locked behind a magical wall, this wall was kept up by this great magical tree. I am fairly sure the wall is invisible, like it's on another plane of existence and the evil has been trying to break through to this world. Well as the tree is dying, one really bad guy gets through first and is now traveling across the land killing people and being generally evil.
Any help would be much appreciated! This was such a good book and I think I just stumbled across it by itself.