The Ash flows from the Mountains of the Morn through the Shadow Lands to the Jade Sea and the Saffron Straits. Asshai sprawls for leagues on both sides at the river's mouth. Much of the Ash's course in the Shadow Lands is through the Vale of Shadows, a valley so narrow that the water is perpetually in shadow. The corpse city of Stygai is located along the Ash in the heart of the Shadow, where two tributaries join.
During daytime the Ash is black in color, but at night it glimmers with a pale green phosphorescence. Fish in the Ash are blind and deformed, and traders thus bring food and freshwater to Asshai. Harmon's On Miasmas suggests that animals are more sensitive than humans to the polluted river. Demons, dragons, and worse creatures are said to inhabit caves in the cliffs above the Ash.