D'ao is the name for the elemental genie of Earth. Herbert Stammelford, a mage, moved a mountain with the help of a d'ao, according to legends.
They were mentioned in the short story "The Last Wish".
“
...once moved a mountain because it obstructed the view from his tower. Nobody has managed to do the like, before or since. Because Stammelford, so they say, had the services of a d'ao, an Earth genie.
”
— pg(s). 251, The Last Wish (UK edition)