The Sea of Sorrow is an unfathomably huge subterranean ocean, located deep within Khyber. On its shores, the ancient ancestors of the Fomor'ren found refuge, and built the city of Vralkast. Vralkast is not the only city state to rest on its shores, either. The Sea of Sorrow is home to pirates, the Vermin States, and a number of other independent city states.
The Sea of Sorrow is home to countless, enormous leviathans that should make it impassable. The Fomor'ren however travel the sea on enormous sailing vessels called Serpentkeels, from which they actively hunt the leviathans, providing food and building materials for the entire city. The sea is also home to the district of Cartilay, which resides beneath the harbour in Sowshore.
The Sea is also a hub for portals to Khyber demiplanes. The Everdelver's Guild of Underplunderers believe that one could get to almost any Khyber demiplane via the sea's shores.