The Bridei Chronicles, book 2: Blade of FortriuJuliet Marillier2005

As the young king Bridei prepares to lead his forces in a major offensive against the Gaels, the princess Ana, a hostage at the court of Fortriu, is sent on a difficult journey north to make a strategic marriage with a chieftain she has never met. Her escort is led by a man she despises: the enigmatic Faolan, Bridei’s assassin and spy. Their ill-fated expedition creates a reluctant bond between Faolan and Ana, which continues as they face unexpected perils at the chieftain Alpin’s stronghold in the midst of mysterious Briar Wood. When Ana discovers a charismatic prisoner who seems bent on remaining within the bleakest of confinement, her attempts to unravel the puzzle are met with a conspiracy of silence.

As Bridei’s forces move down the Glen, it is revealed to those left behind that the king of Fortriu is marching towards not only defeat, but certain death. There is only one messenger who can reach him in time. To employ such an ally will set brother against brother, and endanger what has become dearest to Ana’s heart…