This book is the sixth volume in the mystery series about Sister Fidelma (formerly of Kildare), set in the seventh century in what is now Great Britian. She is red-headed, brooks no fools, and is also an anruth, the second highest rank in the Irish legal system, so she is both a dálaigh (an advocate in the courts of the Five Kingdoms of Éireann), and Celtic religieuse. In these books she sets her skills to uncovering the truth when there are mystery and murder.
The book begins with a young religieuse man being chased and captured. We then move to the Court of Cashel, where Sister Fidelma is asked by her brother Colgú, the King of Muman, one of the four Provincial kings, to go to Laisre, the chieftain of the Gleann Geis in the western Cruacha Dubha mountains. Laisre, who is a follower of the old gods and goddesses, has asked for a religieuse to come from Cashel to negotiate the placing of a Christian church and school in his area for the benefit of his Christian people. Colgú gives his sister the white rowan rod, the symbol of her official embassy power, and the chain of the Niadh Nase, the symbol of her powers at the Court of Cashel. Brother Eadulf, of the Saxons, has not yet returned to the Archbishop of Canterbury, so is free to go with her, so the pair head off on their embassy. When they enter the isolated valley, they discover thirty-three young men dead, in such a way that it appears to be a pagan rite. Once in Gleann Geis, they find that the situation is not just a simple embassy task, but a morass of intrigue and, ultimately murder and conspiracy against Cashel, which naturally Sister Fidelma is bound to investigate (along with the mass murder she already discovered).
This was another very good mystery tale, with many red herrings and such, and I very much enjoyed this book.