Valley of the Shadow — Peter Tremayne

The year is A.D. 666.

Dispatched to the valley of Gleann Geis as an emissary of her brother, the king of Muman, Sister Fidelma has been charged with a formidable task. She must entreat the chieftain of a Druid clan to allow the construction of a Christian church and school in the region.

But before entering the valley, Fidelma and Brother Eadulf find the slaughtered bodies of thirty-three young men--covered in scars from an ancient pagan ritual. As advocate of the Brehon law courts, Fidelma must learn who or what is behind the shocking massacre--before it threatens the peace of her brother's kingdom...