The Mummu are giants of black iron with veins of red-hot blood. Their craftsmanship is unsurpassed, and they are completely immune to heat or cold. Once they become consumed with Torment, they turn into four-armed monstrosities with steel-like skin and serrated claws. Pure chaos writhes around them.
Like some of the other Visages, the Mummu could not exist until the Fallen violated God's decree and made contact with man. Prior to that, those who eventually became Mummu tended to work with the Zaltu and the Mammetum to show humans how to build and master their domains through the example of animals. Adam even managed to build a sling, and use it, but when he forgot how the next day, the frustrated Mummu-to-be decided to rebel. They became the weaponsmiths of the war, forging the tools that their fellow Fallen would use to destroy their enemies. Many were driven crazy after being cast into the Abyss because there was nothing for them to create or create with.