This root was connected to fear and dread in Tolkien’s later writing, most notably in S. goroth as an element in S. Gorgoroth “[Valley] of Terror” as the name of a region in Mordor (LotR/401), as well as in S. Ered Gorgoroth “Mountains of Terror” where Ungoliant dwelled (S/95). The root first appeared in its extended form ᴹ√ÑGOROTH “horror” in The Etymologies of the 1930s (Ety/ÑGOROTH) with a variant ÑGOR-OT mentioned in another entry (EtyAC/GOS). The unextended root √ÑGOR was mentioned regularly in Tolkien’s later writings with glosses like “dread” (PE17/113), “terror, dread” (PE17/154), “fear” (PE17/172) and “terror” (PE17/183). It did not necessarily have an entirely negative meaning, however, as its derivative S. gorn given the sense “revered” in (one possible) etymology of S. Aragorn < Ara-ngorn “Revered King” (PE17/113).
A variant root √NGUR “horror” was mentioned in the Quendi and Eldar essay of 1959-60 (WJ/415), but elsewhere √ÑGUR was generally given the sense “death”; see that entry for detail.
This is a root for “death” words from Tolkien’s later writings, most notably S. gurth “death” as in S. Gurthang “Iron of Death”, the name of Túrin’s sword. The earliest precursor of this name was G. Gurtholfin “Wand of Death”, where the element G. gurth “death” was derived from gu̯rþū́ (✱gwṛþū) or ᴱ✶ngwṛþ- in the Gnomish Lexicon of the 1910s (GL/43). This in turn was a clear variant of ᴱ√GWṚÐṚ “die” from the contemporaneous Qenya Lexicon, with Qenya derivatives ᴱQ. urdu “death” and ᴱQ. warda, though originally this root was just a variant of ᴱ√VṚÐṚ “✱rule” (QL/104).
In The Etymologies of the 1930s this root became ᴹ√ÑGUR with derivatives like ᴹQ. ñuru/N. guru “death” (Ety/ÑGUR), apparently as a state or abstract concept (Ety/WAN). This new root √ÑGUR “death” or “to die” continued to appear in Tolkien’s later writings as well (PE17/95, 154).