Beneath the Hellfurnace Mountains, in the place that surface-dwellers call the Underdark, lies a hemispherical cyst in the crust of the earth, almost six miles in diameter. Strange crystal decays in the zenith of the dome, lighting the sky like an eerie moon and stars, but affording none of the danger to the Underdwellers that the sun would have. This is the Vault of the Drow, home of nearly 50,000 beings, dominated by its capital city of Yethendrin. Here, noble houses of the drow plot and scheme against each other, and many other Underdark races come to trade, labor, engage in diplomacy or intrigue, pledge their loyalty to the drow, or else arrive in chains as slaves.
To the north and west of the city is Darkwater Bay, which permits drow ships to sail the Sunless Sea, engaging in fishing, trade (often in slaves of surface-dwelling races taken from evil empires), and exploration. To the southwest are the Wormwrithings, massive ancient caverns hollowed out by the travels of purple worms, and a source of both mineral and herbal wealth for the drow noble houses. Northeast lies the duergar city of Myrkheim—which has been both a trade ally or a deadly foe of Yethendrin as the ages have worn on—and other major settlements of the Underdark, including deep gnome holdfasts, the Great Fungal Forest, and more.
There are numerous noble houses of drow, but only those who sit on the Council of Eight have true authority in the Vault, vying against each other in deadly intrigue for the coveted position of First House, most favored of the goddess Lolth. A hundred years ago, the Vault was shaken by bloody civil war—the result of a prophecy of a child born with Lolth’s blood and blessing, who would threaten the long-standing absolute authority of the High Priestesss of Lolth. The birth of this spider-headed daughter—whom the drow call the Empyrean—resulted in savage bloodshed and internecine struggle. When the dust settled, two of the Great Houses were rendered excommunicate and destroyed, and a third was heavily censured and forced to intermarry with a lesser house, promoting them in rank. The Empyrean’s faction had triumphed, bringing her House to even greater glory, and a peace accord was reached with the High Priestess, who would remain religious authority over the drow, and cede the secular matters of the management of Yethendrin to the Empyrean. In keeping with ancient tradition, to fill the void left on the Council of Eight, two lesser houses were promoted to Greater status, and have been spending the past century dealing with their new responsibilities and power.