Hounds of War by Nikolas P. Robinson

In 1917, a small group of American soldiers venture into the forests of France, searching for a patrol of Germans seeking to get behind the American lines. Scared and cut off from allies, they discover something far more dangerous than enemy soldiers--something that doesn't pick sides. As days pass in the mist-shrouded ruins of an ancient castle, it becomes clear that none of them are likely to survive. For Sergeant Adrien Demos, the war will take something more valuable than his life, it will take his identity--and if he isn't careful, it threatens to take his humanity.