Ember: Fire and Wind
As its name would imply, the Mirrored Sea is a vast expanse of shiny glass made from fused sand. Created centuries ago by some unknown event (see The Crucible), most of the ‘sea’ is a completely flat, with a few areas of razor-sharp crags and rubble. The glass surfaces create a few difficulties: one, the glass reflects light and heat, creating completely inhospitable conditions; two, supernatural, undead, and heat-resistant scavengers roam the edges of the sea for anything that crosses into the deadly heat; and three, the reflective surfaces generate spectacular mirages.
Most trade routes give the Mirrored Sea a wide berth. However, a defensible overland passage across the glass would be a boon to many. So far, no one has been able to reliably create such a route.