Nan Dungortheb was the dreadful valley in northern Beleriand that ran west to east between the haunted mountains of the Ered Gorgoroth and the enchanted northern marches of Doriath. Ered Gorgoroth was infested with the offspring of Ungoliant, some of which also hunted in the valley below.
The name Nan Dungortheb is difficult to translate. The direct translation is "Valley of the Horrid West" in Sindarin (from nan = "valley", dûn = "west", and gortheb = "horrible"). However, Tolkien translates the name as "Valley of Dreadful Death", though neither of the Sindarin words for "death" (gurth) or "dreadful" (goeol) are apparent. This may indicate a mistake on Tolkien's part, or an extremely prosaic translation of the name.