City of Brass
The City of Brass was a city on Rabiah constructed by Fatima. It was inhabited by Efreeti and Fatima's Brass Men. It is a source of power to planeswalkers, providing any color of mana for a price.
Game mechanics
City of Brass, sometimes referred to as "CoB", was the first land able to produce any color of mana. It was originally printed in Arabian Nights, then later in Chronicles and several core sets. It was removed from Ninth Edition in favor of the painlands, to make the set complement the Ravnica block.