Tolkien gave núya- as an intransitive verb meaning “descend” several times in Late Notes on Verb Structure (LVS) from 1969 as an example of a ya-formative verb, having the forms (a)n(d)úya (PE22/156) or núya- (PE22/163), the latter derived from primitive ✶ndūya- based on the root √ndu “down”. It was contrasted with ta-causative núta- “lower = *cause to go down” (PE22/156).

Conceptual Development: Prior to the 1950s, the verb form ᴱQ./ᴹQ. núta- had intransitive glosses like “stoop, sink” (Qenya Lexicon: QL/68) or “set, sink (of Sun or Moon)” (Etymologies: Ety/NDŪ); see that entry for discussion. However, intransitive núta- had competition from other verbs in Tolkien’s earlier writings.

The Qenya Lexicon of the 1910s also had ᴱQ. nūmeta- or numenda- “get low (of sun)”, both verb forms of ᴱQ. núme “west” (QL/68). This verb appeared in the Oilima Markirya poem from around 1930 and its drafts in the phrase ᴱQ. rána númetar “the moon went down in the West” (MC/221). In notes associated with one of these drafts, Tolkien glossed the verb númeta- as “go down in the west” (PE16/75). The verb númeta- did not appear thereafter.

As noted above, in The Etymologies of the 1930s the verb ᴹQ. núta- was glossed “set, sink (of Sun or Moon)” (Ety/NDŪ). It was glossed “come down” in the Quenya Verbal System (QVS) of 1948, but there it was revised to unta in the phrase ᴹQ. lairesse nihare to tarassi, yu {núta >>} unta hrívesse landannar “in the summer I live in the hills [as a rule], and come down to the plains in the winter” (PE22/125 and note #136). This new intransitive verb appeared a second time in LVS in a rejected page of verbal roots as unta “descend, as of sun or of a man from a mountain” with a half-strong past form ununte “came/went down” (PE22/127 note #152).

The reason for the introduction of 1948 unta- “descend” isn’t explicitly given, but likely Tolkien decided núta- should be transitive/causative, and indeed in Tolkien gave causative ✶ndūtā- “cause to sink” in Common Eldarin: Verb Structure (EVS2) from the early 1950s (PE22/135). It seems núta- remained causative thereafter, but in 1969 LVS Tolkien gave the intransitive/ya-formative form as núya- “descend” (PE22/163), as described above.

Neo-Quenya: Based on this discussion, I would use núya- as the basic Quenya verb for “to descend, *come down”, and thus also useable as “*set (of the Sun)”. However, I think it is worth salvaging 1948 ᴹQ. unta- with the modified sense “descend [below a horizon]”, based on the use of undu for “under, beneath”. Thus I would use núya- to mean “descend [generally or towards a horizon]” while untu- would mean “descend [below a horizon]” and so meaning “*sink, stoop”. Compare also suv- “sink (esp. in water)” (PE22/127).