Counterpart to the root √N(D)IL, this root first appeared in a marginal note from The Etymologies of the 1930s as ᴹ√NDUR or ᴹ√DUR “bow down, bend (down), obey, serve”, an elaboration of ᴹ√NDŪ “go down, sink, set (of Sun)” (Ety/NDŪ; EtyAC/NDŪ). In the Quenya Verbal System of the 1940s, ᴹ√NDUR was glossed “grow, be dark”, no doubt in connection to N./S. dûr “dark” (PE22/103), but later this “dark” word was connected more directly to √NDU “under, down”. Indeed, in a 1955 letter to David Masson, Tolkien gave √(N)DUR an origin distinct from √N(D)U, basing it on an independent root √DUR “to show special interest in things” (PE17/152); there was also an unglossed and deleted root ᴹ√DUR in The Etymologies of the 1930s (EtyAC/DUR). However, in a 1967 letter to Mr. Rang, -(n)dur again meant “to serve” (Let/386).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to keep the connection between √NDUR “bow down = serve” and √NDU “descend”. As for the semantic distinctions between -(n)dil and -(n)dur, see the entry on the root √N(D)IL for details.
This invertible root had a long history in Tolkien’s writings. Its earliest iteration was in a pair of roots in the Qenya Lexicon of the 1910s: ᴱ√OŘO [OÐO] with derivatives having to do with the “dawn”, and ᴱ√ORO “steepness, rising” with which it was much confused (QL/70). The latter had derivatives like ᴱQ. orme “summit, crest, hilltop” and ᴱQ. orto- “raise” (QL/70), and Tolkien mentioned an inverted variant ᴱ√RŌ or ᴱ√ROHO with derivatives like ᴱQ. róna- “arise, rise, ascend” (QL/80). The contemporaneous Gnomish Lexicon also gave it as in an invertible root ᴱ√rō-, oro with derivatives like G. oros “rising” and G. ront “high, steep” (GL/63, 66).
The root reappeared in The Etymologies of the 1930s as ᴹ√ORO “up, rise, high” and ᴹ√RŌ “rise” (Ety/ORO, RŌ). The root was mentioned very frequently in his writings from the 1930s, 40s, 50s and 60s, generally glossed “rise” or “up(wards)”. Thus the root was very well established in Tolkien’s mind.