So another argument with the Diana Wynne Jones podcast.
...in Spellcoats we do have to accept the divine right of kings for the book to work. For the purpose of Spellcoats, the divine right of kings is real.
This a) confuses the historical concept of the divine right of kings with the fantasy trope of the rightful king and b) is the opposite of what Spellcoats is saying about both ideas.
The divine right of kings was the idea that the king was only accountable to god, not to any any earthly force or person. The king wasn't accountable to the common people, nor to the aristocrats, but only to the divine - god, and possibly the church. As a corollary, any opposition to the king was blasphemous as well as treasonous.
If anyone in Spellcoats believes in a divine right of kings, it's the king of the Riverlands. He doesn't really listen to the siblings - who are after all his subjects - unless they convince him it's what the One wants. This is not portrayed as a good thing!
Compare that with Hern, who says to the people, you may choose again later if you want. He makes a bargain out of it: just cooperate with me for three days buy time for my sister to defeat Kankredin, and afterwards, if you want, you can choose a new king. He voluntarily and deliberately makes himself accountable to the consent of the people, which is the opposite of the divine right of kings.
This, of course, rhymes with the things the other Dalemark books have to say about power and accountability. The earls of Dalemark are not accountable to their people, and that's horrifying. That's a fundamental injustice of all three books set in Mitt's time. Spellcoats is not making a different argument!
Which brings us to the trope of the long lost heir or rightful king. , which most frequently is a portrayal where a person, by virtue of their descent or prophecy or birth, is the most suitable person to be on the throne and will be a Good King.
Hern is descended from the Undying. The thing is, so is the king of the Riverlands a direct descendant of the One and Cenblith. All the Heathens are also descended from the One. In Crown, the point is made more explicit: purely based on population dynamics, everyone just about everyone in the Dalemark descends from the Undying and therefore, by descent, has about as much right to the Crown as anyone else.
Legally, his claim to the throne is extremely shaky! He's the oldest brother-in-law of the last king of the riverlands. But the marriage was a sham and he's not actually the eldest. He was wounded with the sword that killed Kars Adon, making him blood brothers, and Kars Adon nominated him as his heir. This is shown to be very tenuous! People are unhappy about it. That's the context of Hern's bargain, just give us three days and afterwards, if you want, you can choose again.
Hern makes himself king, effectively, through being the right person in the right place, and by proving himself through his works in leading the people in the fight against Kankredin.
What role the divine? Well, Tanamil is pretty clear that the Undying don't call themselves gods. And they are neither all powerful nor omniscient, or the One would not have come to be bound in the first place. It's clear that the Undying do shape the story that leads to Hern (and later Mitt) becoming king, but while they can see the future to some extent but they don't control it. Humans have free will. They can bind themselves by their choices, as Hern bound himself with oaths to the one, as Mitt makes the choice in Drowned Ammet to return to the Holy Islands as a friend, not a conqueror.
If there is an idea about kings that you have to accept for Spellcoats to work, it's not the divine right of kings. It is that having a king is a good way for this pseudo-Iron Age society to be organised, and the person who is king should be good at leading people in times of crisis, and it helps to have some relatives who are powerful mages.