The Given Sacrifice — S.M. Stirling

S. M. Stirling's Novels of the Change depict a future without technology, where people must master skills from ages past in order to survive. Now, to ensure that survival for all he cares for, a king faces his greatest challenge in the latest chapter of the New York Times bestselling saga....

Rudi Mackenzie has won the battle that expelled the enemy from the new High Kingdom of Montival. Now he must free the people who live in the state once known as Idaho from occupation by the legions of the Church Universal and Triumphant and pursue them to their lair over the mountains. There he will finally confront the forces behind the Church -- the Powers of the Void.

Yet even a victory will not end the conflict forever. The Powers of the Void are malevolent and infinitely patient, and the struggle is one that involves the entire world. They threaten Rudi not only in the present, but also in the future represented by his children, "rlaith and John. Rudi knows this.

And as his heir Princess "rlaith grows up in the shadow of her famous father, she also realizes that the enemy will do anything to see that she does not live to fulfill her parents' dream....