Alloy, also known as the Burning City and the City of Brass, is the capital city of the elemental plane of Quintessence. Some time in the past, The Chain of Acheron were in service to the Lady of Brass, then the leader of the city, serving as her private army in Alloy and elsewhere. The city is now ruled by Yxmic, a fire dwarf.
Until recently, Alloy was ruled by the Lady of Brass. Her rule was threatened when her general, Xorn betrayed her and turned her armies against her. In response, she hired the mercenary company known as the Chain of Acheron to put down the rebellion. The Chain fought Xorn's forces for three days in the Battle at the Doors of Morning, which ended with Xorn defeated and the rebels having surrendered.
Three years later, while the Lady of Brass and the Chain were in the World Below, a Fire Dwarf called Yxmic with an army of djinn and efreet assaulted and conquered the city. Yxmic enacted a pogrom in the city, sterilizing all those not native to the plane. More than two million of the city's inhabitants fled, reducing its population to a few hundred thousand. The Lady of Brass, upon seeing what had happened in her absence, dismissed the Chain and took refuge in her own castle, whose fortifications Yxmic had failed to penetrate, and flew the castle out of the city and into the Mundane World.
Before Yxmic's pogrom, the city had a diverse population of over 3 million beings from across the multiverse, with the largest non-native population consisting of tieflings. When Yxmic seized power, however, most of the non-native peoples fled, leaving only a few hundred thousand to remain in the city.