Exile - A. J. Calvin - Bog

Andrew grew up in the royal palace of Novania, the eldest son of the queen. He went on to achieve fame and glory as a renowned soldier, and was eventually named commander of the king's army. The kingdom believed he would be named heir to the throne, but he has long known he is ineligible. The king is not his father.
The truth of Andrew's lineage is only partially known to the king; the identity of his father is a mystery that even Andrew is unaware of. He knows only that his father was a dragon-mage, and the dragons have fled to another world. Andrew is a skin-changer, but the laws of Novania forbid his very existence. If the king were to learn the truth of what he is, he would face execution.
The laws are equally hostile to humans born with the Mark of the Magi and the ability to wield magic. Andrew's younger half-brother, Alexander, bears the Mark. The pair keep one another's secrets into adulthood...Until the king dies unexpectedly and Colin ascends the throne.
When Alexander's Mark is revealed for all to see, Andrew is faced with a choice: To watch his brother be killed, or reveal his true nature in an effort to save him from the headsman's axe.