I think this makes for a perfectly decent prologue. That said...
Some explication of the logic behind the "only one heir" rule would be nice. I can extrapolate, but it's so opposite from the usual logic (have as many children as possible to secure an heir for the throne) that it could probably use some explicit explication.
Also, what's this about mingling his seed with mortals leading to no good? Exactly how is he supposed to get ANY heirs out of this deal? I assume avatars of the goddess aren't all that common.
I don't have a problem with Valenth's loyalty, but until the King addressed him I didn't even realize that he was still in the room. Surely he had some reaction to the King killing an unsuspecting priest?
It might be stupid to defy Asharia, who I assumed was a real god. It's also stupid to knowingly throw an entire empire--your empire--into war. He's doing it to save his children. No problems there.
I thought Darion's argument, that "this will only get harder later", made a lot of sense insofar as it's something someone in his position would say (whether it's actually true or not, who knows? no one, I hope). I don't at all understand Chaos's problem with it. (Also, I don't think that this has been sprung on the king unknowingly. I imagine that this has been a tradition, even a law, long before the king's children were so much as a gleam in his eye).