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).
fatanyu noun "hell" (GL:51)