The Ruins of Thumen
Although Sothis was both Osirion’s first and now current capital, it was not always so. During the second age of the Black Sphinx, the slave armies of the Four Pharaohs of Ascension ruled from Tumen. Despite being buried by countless tons of sand, Tumen is still a frequent destination for treasure hunters and Osiriontologists alike.
Sources: Pathfinder Chronicles Campaign Setting