The Edge of the World is a place that is found within the bonus chapter, Journey of Atonement.
The earliest known inhabitants of Edge of the World were a group of people who were shipwrecked by a powerful storm. One of those survivors was an unnamed boy whom father had drowned trying to save him. The group formed an island settlement but neglected the boy because of a shortage of food through the winter. The boy became friends with the animals and chose to live with them, even remaining behind when the group was rescued.
When the boy grew into a man, he deconstructed the settlement and reused the materials to create a temple that in time would be dedicated to storing magic too dangerous for humans. He became the first known Guardian. Many more Guardians would follow him over the centuries, until the position was passed to man known only as the Old Guardian.
The Edge of the World was sought out by Kai to ensure that the Tinderbox's powers were kept away from anyone who might come across it.
The Temple at the Edge of the World (from Journey of Atonement)
Long ago, a ship was wrecked by a powerful storm at the very Edge of the World. Among the many survivors was a little boy whose father drowned while saving him. The survivors formed an island settlement, but throughout the winter, food was scarce and heat was rare, and no one wanted to take care of the child. The child became friends with animals, and chose to live with them instead of humans. When the survivors were rescued at last, the child remained behind on the island. Over a period of years, he deconstructed the settlement and reused the materials to create a temple. He had become a grown man by the time he slept in his temple for the first time, and that night he had a vision. He would dedicate the Temple at the Edge of the World to safekeeping the magic too dangerous for humans. He became the first Guardian, but not the last; many would sacrifice their freedom for the greater good in centuries to come.