The book is set fifteen years after the trilogy. Long enough for Lord Nnanji to conquer half the World in the name of not-barbarianism. Long enough for Wallie Smith / Shonsu to have a bunch of kids and, well, he's still a great swordsman but maybe not as fast as he used to be, eh?
Then assassins climb over the wall for both Wallie and Nnanji, separately, on the same night. Nnanji is wounded. Wallie is not, but he knows that wizards are on the move, somewhere out on the River. And the wizards are supposed to be good guys now. But then, so are the swordmen.
The fun of this series is the mix of scintillating swordwork, rigid code-of-honor barbarian culture, and the clever twists that people come up with to do the right thing without either violating their cultural standards of honor or getting their guts scintillated. Also: the layer of pure-fun adventure on top of a story that's really about the birth of a civilization.
This fourth book holds up the bar. It concerns the next generation -- Wallie's adopted kid Vixini and Nnanji's heir Addis -- as much as the original characters, so we get both wild younglings and their somewhat-level-headed elders. And it plays scrupulously fair with the god's promise, at the end of the trilogy, that the age of miracles was over.
When I say "scrupulously fair", of course I mean that the god cheats outrageously. But within the rules! Like I said, clever twisting.