At the beginning I suggest you that you open the world map, so that you can determine your destination. The Tree of Life is located in the northern part of [Forge Lands] (the above screenshot). Best to begin a your journey in [Stonefather's Vale], where you have just fought the Guardian.
After you reach the following global and use the thin passage that was unblocked by the "healed", Guardian (the above screenshot).
Now, you need to get through the location [Green Basin]. There are no enemies here, so follow the linear path. After you reach the location. [Tree of Life]get off the horse and approach the tree. This will result in several cut scene. It's that and this mission. Furthermore, you will be transferred to the [Kingdom of the Dead]. The reward for the completion of this mission are: 1440 experience points and 7000 gold pieces. Also a new mission. [the Lord of Bones] will start automatically.
[TIP: as I have just said you don't really need to be concerned about the unfinished business in the[Forge Lands]. In the [Kingdom of the Dead] there is a portal that makes travelling between various locations in the game world possible. Also, previously began missions will not be prematurely close so you may return to doing them anytime you want.