The Well of Shades [first edition]

Juliet Marillier continues the epic fantasy begun with The Dark Mirror, which Interzone called: "A fascinating evocation of life in Pictish England and an emotional roller coaster of a story." King Bridei is a man with a mission. His wish to unite his kingdom seems almost within his grasp but there are forces working to undo his dream. He sends Faolan, his most trusted advisor (who is also a master assassin and spymaster) out into the world to ferret out the truth of who is friend and who is foe. Along the way Faolan will uncover many truths. Some may hold the key to Bridei's future. But more important, they may unlock the secrets that Faolan has held deep within his soul for decades. And offer him the chance of redemption.