Mercury, according to the consensus, is a barren, airless world pockmarked by thousands of craters. This planet has a slow rotation (59 earth days is one Mercury day). A Mercury year by contrast (the amount of time it takes the planet to revolve around the Sun) is 88 days. There is no volcanic activity, and by all appearances is a dead planet.
The Umbral equivalent is similar difficult to inhabit, thanks to its proximity to the Sun. It is connected to the Shade Realm of Correspondence. At one time the Ahl-i-Batin had a Chantry on this world. The Mages had slowed the worlds rotation so that the settlement, which was built in a crater, was always on the edge of twilight. Now the whole surface of the planet is susceptible to the extremes of temperatures, 700 degrees Fahrenheit on the day side, -274 degrees on the night side.
Mercury once held the largest Batini Chantry, known as the City of Brass. It took its name from the odd color the sun turned its limestone and mud buildings. The Realm is lost now and all its portals are closed.