Aether-gold, also called "The Breath of Grungni", is a magical metal found in the clouds of the Realm of Chamon. It is naturally lighter-than-air, but in large volumes it can also change the weather and can cause madness when it rains down from the clouds.
The Duardin Kharadron Overlords harvest it by vacuuming up the clouds and using alchemical processes to turn it into ingots.
The physical properties of aether-gold are not fully known; the Kharadron Overlords use it to lift their ships and sky-ports, but they are researching other uses for it.
Monsters such as harkraken, chimera, megalofin and Skaven are also attracted to it, leading to conflicts between them and the Kharadron Overlords.