|Archduchy of Sirmium|
Source: Cheliax, The Infernal Empire, pg(s). 3-4
Named for the Sirmium Plains that expand across much of the region, the Archduchy of Sirmium encompasses southeastern Cheliax from the Iseld River in the west to the Aspodell Mountains in the east and Keld River to the north. The Inner Sea forms its southern border, spanning the Bay of Solva, Bay of Deng, and Cape Erebus. Its capital is Ostenso, located on the archduchy's southeastern coastline, where House Henderthane maintains its rule over the region under House Thrune.