Dao are one of the four genie races of the multiverse of Dungeons & Dragons, being associated with the Elemental force of Earth. Like all genies, they are natives of either the Elemental Planes, specifically the Plane of Earth, or the Elemental Chaos, depending on whether you look at them from the perspective of the Great Wheel or the World Axis.
The dao are the most avaricious of all the genie races; residents of the Plane of Earth, they constantly seek to mine the gemstones and precious ores of their planes, often extending their operations into the Material Plane or the Plane of Minerals to pursue their voracious hunger for more and more mineral wealth. They are the only genie race whose love of enslaving mortals and elementals to serve them matches - or even exceeds - the efreeti. The dao occupy a portion of the Plane of Earth known as the Great Dismal Delve, which is made up of all the various maze-like fortress-citadels of the dao. Their ruler, the Great Khan, has the biggest and most impressive mazework citadel of all, a labyrinthine city called the Sevenfold Mazework, which links to all the other dao mazeworks, with the Great Khan's personal fortress being the Hidden Fulcrum of the Dao, located at its center.
Dao are reviled by djinni and marids; only the efreeti are willing to trade with them, since the dao are master producers of raw ore for the fiery genies to smelt and have the largest slave market in all of the planes. Despite their use of slavery, dao are the most driven and industrious of the genie races, albeit focused in the fields of pursuing greater wealth. Their slaves are gold-plated, their gold is diamond-studded, and their diamonds are hollowed out to encase shavings of platinum and saffron. These genies are diligent, hard-working, and driven to complete their tasks to the best of their abilities. Dao take pride in a thing well-made and a plot well-planned. Mortal agents sometimes employ dao to build fortifications and palaces, for their engineering skills are second to none. When constructing something, the malicious nature of the dao is submerged in the industry of the work.
Though dao are evil, they are also honorable in their own way. If shown kindness and fairness, a dao will return them in kind (taking everything else it can get as well). Dao make excellent advisors, particularly if they know that by helping a mortal they can cause harm to other mortals. These genies hate enslavement as much as the efreet, and will seek out sha'irs who have imprisoned their brethren - either to slay the sha'irs or to take them as slaves.
Male or female, dao are powerfully muscled individuals. Their polished skin is the color of earth, sand, or granite, and their finger- and toenails are made of a durable but lustrous metal. The fingers themselves are wide and pudgy, even if the dao assume other forms. Both sexes are bald and free of body hair. Males do have facial hair, worn in mustaches and angular beards. All dao enjoy adorning themselves with jewelry, such as earrings, bracers, chokers, nose rings and studs, ankle bracelets, rings, and bells. Many don shirts of lamellar plates.
Dao do not need to eat or drink. They can fast for years without significant detriment to their abilities. They also can slow their breathing to a point that enables them to remain buried beneath tons of debris and be unaffected. (They are still vulnerable to poisons and air-based attacks, however.) Despite such powerful constitutions, dao enjoy the sensations of life, and often partake in hedonistic revelry. For example, dao believe that powdered gems, gold dust, and gold leaf heighten the experience of eating - devouring them as mortals might use a precious spice.
Of all the genies, daos have the weakest ability to grant wishes, at least in Advanced Dungeons & Dragons; according to their expanded lore from the Al-Qadim setting, not only can a dao only grant 1 wish a dao, but they can only grant it to true natives of the Material Plane (even planetouched are too "magical" to benefit), and the wish will always carry a hidden sting in its tail, even if the dao doesn't want it to! Noble Dao can grant three of these perverted wishes per day. In Dungeons & Dragons 3rd Edition, this was changed to dao simply being able to grant 3 Limited Wishes per day (but only for nongenies).
Dao are greedy, malicious genies from the Elemental Plane of Earth. They adorn themselves with jewelry crafted from precious gems and rare metals, and when they fly, their lower bodies become columns of swirling sand. A dao isn't happy unless it is the envy of other dao.
All That Glitters: The dao dwell in complexes of twisting tunnels and glittering ore-veined caverns on the Elemental Plane of Earth. These mazeworks are continually expanding as the dao delve into and reshape the rock around them. Dao care nothing for the poverty or misfortune of others. A dao might grind powdered gems and gold dust over its food to heighten the experience of eating, devouring its wealth as mortals consume a precious spice.
Lords of the Earth: A dao never assists a mortal unless the genie has something to gain, preferably treasure. Among the genies, dao are on speaking and trading terms with the efreet, but they have nothing but scorn for djinn and marids. Other races native to the Elemental Plane of Earth avoid the dao, which are always seeking new slaves to mine the mazeworks of their floating earth islands.
Proud Slavers: The dao trade for the finest slaves that money can buy, forcing them to work in dangerous subterranean realms that rumble with earthquakes. As much as they enjoy enslaving others, the dao hate being enslaved. Powerful wizards have been known to lure dao to the Material Plane and trap them in the confines of magic gemstones or iron flasks. Unfortunately for the dao, their greed makes it relatively easy for mages to cozen them into service.
The origins of the dao in the World Axis are unknown. What is known is that they are cruel, rapacious, and greedy creatures, constantly slaves to further their ability to strip mineral wealth from the infinite expanses of the Elemental Chaos. They are... pretty much identical to their Great Wheel counterparts, save for the art implying they are more monstrous; giant, earthen-hued humanoids with glowing eyes, hairless bodies, and horn-like stony growths armoring their skulls and joints.
The dao, like the marid, are the "youngest" of the D&D Genie races. They only appeared in Advanced Dungeons & Dragons, making their debut in the article "Featured Creatures" in Dragon Magazine #66, before seeing official print in the adventure Lost Caverns of Tsojcanth and the Monster Manual II.
They first made their appearance in AD&D 2e under the "genie" heading in the Monstrous Compendium Volume One, which was then reprinted in the Monstrous Manual. The noble dao and the Great Khan of the Dao appeared for the Al-Qadim setting in the Monstrous Compendium Al-Qadim Appendix. The dao also appeared in this setting under the header "Genie of Zakhara" in the Land of Fate boxed set - this sourcebook offers the largest amount of lore that the dao has ever enjoyed, and is far more informative than the Monstrous Compendium/Manual entry.
History would repeat itself, with the dao losing its place of prominence in Dungeons & Dragons 3rd Edition; instead of appearing in the Monster Manual, like the djinn, efreet and even jann did, it wouldn't make its debut in 3e until the release of the Manual of the Planes.