From the website:
Battletome: Kharadron Overlords introduces a new duardin army into the worlds of Warhammer Age of Sigmar. A militaristic, mercantile society detached completely from the other branches of duardin that populate the Mortal Realms, they treat war and trade as much the same thing – a means to amass wealth; and they excel at both. More than anything, they seek aether-gold – this lighter-than-air element is their society’s lifeblood, holding their cities aloft, fueling their ships and powering their elaborate armour. Great sky-fleets of warships descend from dozens of sky-ports when a rich seam is discovered; the Kharadron Overlords will fight bitterly to accrue this vital substance, and the status it brings.