The Underdark region beneath the surface of the Lands of Purity is called Onvorn. Like the surface region above, a large portion of this region is covered by a freshwater sea. This one though is in the black reaches of the Underdark and named Ebonmurk. Because of this freshwater sea and the sustenance it affords, many creatures dwell across Onvorn.
One trade route between Onvorn and the surface begins at the Tara-Har and then moves through the Black Spiral until they the Ebonmurk. Another major route into the region from the surface is via the Vortices of Khushnír.