Shintetsu no Agartha

From Fallen Syndicate: The Magane infection turns humans into "Magane," monsters of corrosive iron. The Magane prey on others by devouring or infecting them. They consume entire villages, leaving no survivors of their destruction... until Alma and his group of friends appear. They possess powers far above those of a normal human: the power to kill the Magane. Alma and company save the young Tenkou from infection, and he soon becomes their guide to the legendary "Agartha of Shintetsu."