Centuries ago, the fairies were driven north, where they lurk behind the Shadowline. They want their lands back. The humans living in Southmarch are blithely unaware that the Shadowline is drifting purposefully southwards, being preoccupied with their own politics. The king is being held hostage by a treacherous southern neighbor. The oldest prince is murdered shortly after the book opens, leaving the teenage twins, Briony and Barrick, as the regents.
Briony manages to rise to the occasion, but her half-crippled brother starts cracking under the strain. The book follows several other characters, notably Chert, a hobbit-like creature who adopts a strange boy that he found wandering near the Shadowline, and Qinnitan, the newest wife in the harem of the Autarch, far to the south of Southmarch.
Williams juggles the various storylines fairly effectively, building a new world of fantasy. His characters have plausible motivations and problems. Perhaps he could have done it in fewer pages. This is the first book in a projected trilogy. Is there a law requiring fantasy books to be be huge?