The Well of Shades by Juliet Marillier
In the sixth year of Bridei's kingship, he sends his chief assassin and spy, the Gael Faolan, back home to Ireland to investigate reports of a powerful Christian cleric whose charismatic leadership may prove a threat to the stability of Bridei's pagan kingdom of Fortriu. Years ago, the action of a cruel chieftain caused the young Faolan to deal with a shattering blow to his own family, forcing him to leave home for good. Now he must return and face his demons. . .
'Juliet Marillier is among the most skilled of fantasy writers' Sara Douglass