Corus the Champion is the story of second sons. While the story continues the tale of all the Barlow brothers, it focuses on Ewan and Corus himself. Their stories highlight the difference between surrendering something that is not yours to give in order to get something that you want, and sacrificing something precious of your own in order to save others.
Corus was a mighty warrior. It was his duty to help protect the Hidden Lands. It was his fate to search for the sleeping king. Then he was seduced by Nemesia and would have given her the secret of the Pillar Map. To keep him from this Sorge tricked him, gave him to the Fey, who later gave him to the evil Kr'Nunos. Corus was kept prisoner, tortured and taunted, kept from his duty and his fate. He prayed and wished for freedom, but when at last it is given to him, he can't bring himself to take it because of the shame of what he did. His shame strips him of his ability to do what he was meant to do. His past failures keep him from doing what he was meant to do.
In contrast to his actions are the actions of so many others who sacrifice themselves to save him. There is Barsonici who truly understands "Light brings freedom. Hope is vision. Vision is light." He sacrifices himself to allow Sorge to escape from Hel with Corus. There are the Bird Men who sacrifice themselves to fly Corus to safety.
And there is Ewan, who sacrifices his gift of Song in order to save his friends and discover what happened to Corus. Ewan gives his gift to the Queen of the Fey, leaving him empty, stripped of his ability to play music. But it is this sacrifice that allows him to fulfill the destiny that Corus should have had, to find Artorius and wake him up.
While Corus rouses himself by the end of the story, it is still unclear what his intentions are regarding the king. On the other hand, while Ewan is still suffering because of the loss of his gift, he is the true champion of the story.