Sindarin name of Morgoth’s fortress Q. Utumno (MR/382) translated “Dark Pit” or “Hell” (RC/297), probably a derivative of the same primitive form ✶Utupnu as its Quenya cognate (MR/69). It was also the name of a plain in Mordor (LotR/928).
Conceptual Development: In the Gnomish Lexicon from the 1910s this name was G. Udum or Uduvna (GL/74), probably derived from the same primitive root ᴱ√TUM(B)U as its (early) Quenya cognate ᴱQ. Utumna (QL/95, LT1A/Utumna). In The Etymologies from the 1930s, N. Udûn had essentially the same form and meaning as the later Sindarin name, though at this stage its primitive form was given as ᴹ✶Utubnu (Ety/TUB).
The vale in Mordor was first called N. Narch or Narch Udûn (SD/34, WR/438).
According to Christopher Tolkien, this was the initial name of the valley of Udûn in Mordor (SD/34), also appearing as Narch Udûn (WR/438). This name seems to have the same word narch seen in Narchost (the second of the Towers of the Teeth), but it isn’t clear whether J.R.R. Tolkien intended these names to be related.