Kabaneri of the Iron Fortress

Streaming video - 2016

At a time when the industrial revolution was carrying the world into the modern age, a horde of undead monsters suddenly appeared. They could not be killed unless their hearts, which were protected by a layer of iron, were pierced. Humans bitten by them would come back as undead monsters themselves. These undead, which would later be known as "Kabane," exploded in population until they finally overran the world.