The World Tree is the tree that supports all the nine world in the mythology of the Northern Pantheon? of gods. The life and death of all things is written on a branch on the tree.
From the Books: "I spent the day examining my guilt. Though the Trickster had helped me break my oath, I still held the shame of oathbreaking. This shame filled me all the more since I had not died to fulfill the oath. Bedarth made me promise to live with that guilt. He made me swear to the World Tree that I would live and fulfill the life written for me in its branches. When I honored that promise, he poured all of his knowledge that he could into me." - ALMD, p. 298