A verb for “come again” appearing in its future form entuluva in the sentence aurë entuluva “day shall come again” from The Silmarillion (S/195). It is a combination of en- “re-, again” and tul- “come”. It was also translated “return” as an element in the ship name Entulessë (UT/171) and in drafts of the Löa Yucainen poem in the phrase loar! loar aluvalle koiveanyo entule naina “years years never again in my life will you return upstream” (CPT/1296).
Conceptual Development: The earliest Lost Tales had a phrase ᴱQ. i·kal’antúlien “Light hath returned” (LT1/184), and this verb ᴱQ. antulu- seems to be the earliest precursor to entul-. Notes on The Feanorian Alphabet from the 1940s had ᴹQ. at-tul- “come back, return” (PE22/047), using a different prefix at(a)- for “back, again, re-” (Ety/AT(AT)). In Tolkien’s later writings, at(a)- specifically meant “a second time“ or “double” (PE17/166), and the prefix en- “re-” was introduced as a prefix for general repetition; see that entry for discussion. The final version of the Löa Yucainen poem instead had nantul- for “return” [= “come back”].
A noun for “return” attested as the name of a ship: Entulessë (UT/171). It seems to be an abstract noun based on the verb entul- “return, come again”.