Dragon and His Grandmother

When three soldiers leave the battle to hide in a field, a dragon rescues them...but it comes with consequences.
Dragon offers them safety only if they serve him for seven years. To be set free they must answer a riddle. Will the soldiers gain their freedom, or will they be servants of the Dragon forever?
This is a grim retelling of "The Devil and His Grandmother," an 1800's English tale adapted by the Brothers Grimm.