Banished Lands are going down because King Nathair has three treasures in his grasp which means that there are only four remaining. After getting the Seven Treasures, the king will be able to open the portal through which the devils from the other world would be able to come to the Banished Lands.
There will be total chaos and bloodshed after that and there would be no one to stop any of this. Corban is in a broken condition because he has lost his friends and in order to survive, he has to look for new allies. If a strong alliance is not formed then Corban and the Banished Lands will end in smoke within no time. It is a story that will show the strength of a man who is trying to rise after falling from a great height.