Here is
a draft:
Er hondonen yúyun véto, núriessen morniéno, hiruvatyë nár melmëo ar astaro i nutë vet oialë.
"With one heart for both of us [two], in [the] depths of darkness, you will find [the] flame of love and loyalty that ties us forever."
er hondo-instrumental
yúyo-dative
ve-dual-genitive, *
núrië-plural-locative
mornië-genitive,
hir-future-2nd_person_familiar
nár melmë-genitive
ar astar-genitive
i nut-aorist
ve-dual
oialë
Each word in the gloss above leads to the page of the respective word on
Eldamo (the most exhaustive and current lexicon of
Tolkien's invented
languages, maintained by Paul Strack).
I had to paraphrase "yours and mine", because the attestations of
Quenya independent pronouns like this are scarce and their derivation in Late (post-LotR) Quenya debatable.
núrië is a neologism (i.e. word not attested in Tolkien's published writings, but reconstructed by fans), but its derivation mirrors that of
tárië "height" and is, in my opinion, as safe as a neologism can be.
hiruvatyë "you will find" uses the familiar second person ending, i.e.
the one that you would use for your beloved. (The form with the formal ending would be
hiruvalyë, as used by
Galadriel when addressing
Frodo.)
The definite article is normally not used in Quenya when noun is already defined by genitive.
What I gave above is a possible translation which is quite safe
from the point of view of our current knowledge of Quenya (this is always a necessary disclaimer), but it might be possible to experiment with alternative ways of expression or with word order, which might perhaps allow to achieve a better rhythm. It is also possible (and always advisable) to ask other people, so that they might come up with different options. (I would recommend the community
Vinyë Lambengolmor on Discord, of which I myself am a knowledgeable, but by no means the only or the most knowledgeable, member.)
In any case, I would recommend to wait a bit, so that I myself can revisit my translation, say, tomorrow. It never hurts.
