The Forbidding Sea is a largely unexplored and uncharted sea along the eastern coast of the Eastern Kingdoms, sometimes noted by travelers on the coasts of the Hinterlands, Arathi Highlands, and Swamp of Sorrows. With the Burning Crusade expansion, the Eversong Woods were also included.
With patch 3.0.2 and the release of Wrath of the Lich King, there is now a coastline in the far east of Eastern Plaguelands near the starting area of the Death Knight. Most of these waters are made impassable by many mountain ranges on the long eastern coast of the continent, East of Khaz Modan.
It is possible that the Forbidding Sea "merges" with the Veiled Sea.