Traitors' Gate. Book Three of Crossroads

Reeve Joss is struggling to defend a country ravaged by the assaults of twin armies. His men now patrol a land of burning villages and homeless refugees as Joss tries to separate traitor from friend. The Reeve's thoughts are also plagued by the intriguing Zubaidit, pleasure-giver, spy and temple-trained assassin. But Zubaidit is focused on a dangerous mission, her target being warped Guardian Lord Radas.
His death would leave the invading militia in chaos, but the old tales tell truly of the Guardians' immortality - and of the powers they now wield to twist the hearts of men. Joss's nights are also troubled, disturbed by dreams of Marit. His lost love has returned from death to become a feared Guardian herself, but Marit rejected the corrupt temptations they offered. She now seeks others of her kind, praying some are yet uncontaminated by the blight on the land - and have the will to fight it.