While the previous trilogy stars Morgan, King Eelson's champion and mentor, this series stars Kelson.
Three years after the events attending upon Kelson's coronation and its immediate aftermath, former Archbishop and Primate Loris escapes from the monastery in which he is being held with the aid of old enemies of Gwynedd. To say more about the plot would involve major spoilers, so I'll say no more than that the plot is tight and derives directly from the personalities of the characters. There are several shocks along the way, and the story sets up the next book very well. The ending of the story is very emotionally affecting.
The world of the series is obviously heavily inspired by the Catholic Church, and partisans on both sides of the conflicts are clearly sincere in their beliefs. This does not prevent there being obvious good and evil; there are few more evil than true believers lacking conscience.
We get excellent character development for Kelson, Morgan, Donall, and Kelson's blood-brother Dhugal MacArdry, and very interesting events involving all of the members of the family of Dhugal's estranged uncle Sicard.
The writing here is notably more mature than that in the previous trilogy, with more believable characters, better plotting, and more interesting politics.
While it wouldn't be necessary to read the previous trilogy, reading it would benefit a reader's understanding of the politics and personalities of this series. With or without reading that series, though, this is highly recommended.