Kinda important what the king can do if I don't decide to revise it out. Could be a pure constitutional monarch with a figureheard; could even rip off Cordwainer Smith and have them all loyal to a king who vanished centuries ago, as there's no knowing, he might come back. Could be an absolute monarch, though not likely in other details of the society. Could be -- a magical position where the big job is keeping the land safe and prosperous.
A magical position, like that of absolute monarch, requires some capabiliities of the monarch -- though very different ones -- probably. You could just have a spell where the land is dependent on their being a person of the royal blood. (Which would explain why the evil usurper locks the monarch in a tower and blinds him without kililng him.) But such inklings as I have had point to this king needing to be a wizard, though not a great master wizard.
Still up in the air on it. It's only in the latter books that it may come to the front.