Green rider

On her days-long journey home, Karigan Gladheon trudges through the immense forest called Green Cloak. She is leaving school, or, more accurately, she has been asked to leave. "Suspended indefinitely," the dean said. And for what? Challenging and thrashing Timas Mirwell in the practice ring. Karigan felt no remorse when she stood over the heir of the lord-governor of Mirwell province, her wooden practice sword at his throat. He had humiliated her, the daughter of a "mere" merchant, since the first day of school. But now, as she faces the long road home, hoping to arrive in Corsa before the dean's letter does, she wonders if she should just hire herself out on a merchant barge as her father did and stay away for good. She has too much pride, too hot a temper, and a penchant for danger - a life of travel, adventure and commerce will undoubtedly suit her just as it has her father.
But before Karigan's ponderings can become a definite plan, her thoughts are interrupted by the clattering of hooves, as a galloping horse bursts from the woods in an explosion of branches, the rider slumped over his mount's neck, impaled by two black-shafted arrows. As the young rider lies dying on the road, he tells Karigan that he is a Green Rider, one of the legendary messengers of the king, and that he bears a "life and death" message for King Zachary in his horse's satchel. He begs Karigan to carry his message, warning her not to read it, and when she reluctantly agrees, he makes her swear on his sword to deliver the message "for love of her country." As he bestows upon her the golden winged-horse brooch which is the symbol of his office, she seems to hear the ghostly sound of great wings beating the air, of silver-shod hooves galloping...and of invisible lips murmuring, "Welcome rider..." But beside her on the road, the messenger only whispers, "Beware the shadow man!" with his dying breath.
This promise given to a dying man changes Karigan's life forever. Pursued by unknown assassins, following a path only her horse seems to know, and accompanied by the silent specter of the original messenger, she herself becomes a legendary Green Rider - the magic of this office transferred to her with the original rider's brooch. But what is happening in her land that requires a life or death message for the king? What does the message contain that is so vital someone is willing to kill for it? And who is "the shadow man?"