The City of Brass was a chantry of the Ahl-i-Batin located on Mercury, from which they researched the Shade Realm of Correspondence. It was established in the 1500s, using powerful magic to slow Mercury's rotation so that the same side always faced the sun; the chantry was located in the narrow band of twilight between the day and night sides, and its name came from the color of the the permanent sunrise against the simple clay and limestone buildings.
In 1847, the Batini in the City of Brass informed the Hermetic Fors Collegis Mercuris on Mus that they had identified a potential traitor in their midst; while some of the population of the City of Brass were evacuated to the college, the rest were confined to the surface and placed on a kind of lockdown. After months without communication from the surface, the Hermetics noted that the rotation of Mercury was speeding up: soon, the City of Brass was exposed to the full force of the Sun and incinerated, taking with it the remaining Batini and much of their knowledge of the Shade Realm.