Mostiae Island is the second largest island in the Northern Isles, and like the largest, Iama Island, was formed through volcanic processes, with two massive extinct magma chambers lying under its two highest points.The word "Mostiae" is from the language of the Northern Isles and, though a direct translation is impossible, refers to a defensive ring. Indeed, when the highly independent population of the Northern Isles made contact with the rest of Ostentia, they feared that they would be attacked and turned it into a defence outpost, protecting Iama Island. These fears proved to be missplaced as mainland Ostentia, like the Eronam Empire that came before it, had no interest in the little islands. Many of the defensive towers have since crumbled while those that remain are considered tourist attractions.
The primary industry of the island is fishing, with the seas around the island rich with fish, including some unique to the area. However, over the years there have been several disputes between the fishermen of Mostiae Island and Iama Island, and as a result Mostiae's fishermen were forbidden from fishing in the areas closest to Iama Island, which are the best fishing grounds in the Isles, though in return the fishermen of Iama Island agreed to give a yearly payment to Mostiae. In recent years, relations have generally been good between the islands, and were further strengthened by the joint demands for separation from the Union.