This armor isn't particularly thick or strong, seemingly more for appearances than anything else. It has one notable defect - at some point the wearer was stabbed in the back, leaving a large gouge in the metal. It doesn't show it's true properties until it's put to actual USE, however. If the wearer is injured in combat, any enemy in the area (not just the one who struck) will receive the same injury, with the same severity.
You found it on a corpse in a field of similarly armed and armored stiffs, all of whom ALSO seem to have been stabbed in the back, but without any mark at all on THEIR armor. I guess someone acted rashly.