After finding The Dark Mirror somewhat lacking, it took me awhile to get around to reading its sequel, Blade of Fortriu, but I wasn't disappointed; surprisingly, this middle book is actually the best of the trilogy. It begins five years after Bridei's rise to kingship, just as he's about to go to war with the Gaelic kingdom of Dalriada because... well, because they're Gaels and therefore should go back to Ireland. He sends off the hostage princess Ana as a bribe for the mysterious chieftain Alpin of Briar Wood, and with her his Gaelic bodyguard Faolan, chiefly because he's causing King Bridei cognitive dissonance (see: reasons for war). This book follows both sets of characters, alternating between the adventures of Ana, Faolan, Alpin, and Alpin's brother Drustan, and the military situation with Bridei, in addition to a couple of subplots.
Blade of Fortriu starts off amazingly well; for the first 200 pages or so, it was my new favorite Marillier book. The plot is compelling, and Ana and Faolan make much more interesting characters than Bridei and Tuala did in the last book... for that matter, Bridei and Tuala have improved, as his struggles with the reality of war and hers with her frighteningly precocious toddler make it easier to relate. The problem is that the story doesn't quite follow through on its promises as it progresses. Alpin begins as a rather intriguing character with some complexity, only to devolve into a one-dimensional villain; Ana's love life is interesting at first, only to be glossed over in the second half of the book, as if even the author thought she'd chosen the wrong man. Ana and Faolan's story is by far the more interesting for most of the book, but when it starts to lose intensity, the slack is only partially picked up by Bridei's plot, which is action-oriented but predictable.
Still, this is an enjoyable read, and if you liked The Dark Mirror (or your reasons for not liking it included Bridei's annoying perfection or the lack of action inherent in watching children grow up), you'll probably like this one. Blade of Fortriu could probably be read as a standalone; the setting and most of the characters are introduced in its predecessor, but the plot is new. Be warned, though: the end of this one screams "sequel!!" so loudly that you might as well just get The Well of Shades too.