Shadow Gate: Book Two of Crossroads by Kate Elliott
Marit was pretty sure she had been murdered. She recalled the assassin's dagger, thrust up to pierce her heart. A certain path to a swift death, as any reeve would know. Her dying vision had shown her the next world but her spirit had not made the journey. She woke alone, sprawled on a Guardian altar with only a cloak for a covering. Her eagle was dead. The Guardians had ruled the Hundred, but they had long disappeared from the world, leaving the reeves to administer justice in their stead. But this peace has been shattered by a desperate army, slaughtering all in its path. And its leaders are shadowy undead, wearing the cloaks of lost Guardians ...