Impossible Odds by Dave Duncan

From the publisher (source: The Internet Book List):
Deposed by a foul usurper, the Grand Duke Rubin is on the run and in desperate need of protection. While the King has decreed that new Blades will be magically bound to the guard, not one of the rough youths being readied at Ironhall yet possesses the seasoning and maturity to accept such an assignment. Left with no choice, the Grand Master approaches two with an offer of early bonding and probable death: Ranter, strong but arrogant, dense, and rude, and Ringwood, eager and impetuous, who might make a fine Blade -- someday. Since they will need much help -- and luck -- to survive what better, more skilled Blades have not, a third is enlisted into their threadbare ranks: the candidate Bellman, who, though barred from the Blades by injury, may have some small talent for espionage.

Joining the Duke's entourage along with a courageous and prescient White Sister named True, the trio of would-be champions begins an astonishing journey filled with trials and horrors, intending to restore a rightful ruler to the throne or die in the process. But before them waits an army of the dead -- a dark fraternity of shadowmen, savage and unstoppable, who slaughter in the service of a despicable fiend driven by greed and the blackest of hearts. And the Duke, whom the Blades must protect to the last drop of their lifeblood, is not the liege they imagined -- but rather the guardian of strange and twisted secrets ... and a hidden identity that threatens to plunge their noble enterprise into total confusion and even graver peril.

This book is part of the "The King's Blades" series.
Here are some other books from this series: