though wise men at their end know dark is right

please excuse my bad translation so far:

though wise men at their end know dark is right, because their words had forked no lightning they do not go gentle into that good night

quamquam sapientes apud fine eorum sciunt tenebrae est iustae, quia verba eorum non seperaverant fulgur, ii non eunt lene in illo bona noctem

Hemo Rusticus

J. Wellington Wimpy
Rather difficult choice for translation, друже мой. Commendable, to say the least!

First part is solid enough, although I must say that if 'apud' were the right word to use (and I don't think it is), it would take the accusative, thus apud finem. It seems that the word finis by itself was used in this way 'after the Augustan period', so that you may rightly use it possibly with the addition of supremus 'uttermost'. Also, instead of your eorum, which would refer to some other entity, we need a form of suus 'one's own' (= R свой, т.е. Мудрые знают во своем конце...)

Quamquam sapientes (supremo) fine suo sciunt...

For the next part (... that dark is right), you'll need to familiarize yourself with the idea of the indirect statement, which requires a special formula.

quia verba eorum
Again, instead of your verba eorum (= R их слова/слова их), you must use suus (свои слова) because they are not the 'words' of some other people, but of the 'wise men themselves'.

I understand 'fork' not to mean 'split' but rather 'cause to be split', as in 'draw in the lightning (because of the words' greatness/height/grandeur)'. I really don't know how to translate that.

ii non eunt lene
This is very good! So long as you haven't misconstrued lene as an adverbial form belonging to the wrong declension (as if from < *lenus, -a, -um). Rather, the word is lēnis (3rd decl.), & here you have the neuter sg. functioning adverbially. The last issue is an easy fix.
in illo bona noctem
All of these words must agree (feminine, singular, accusative), so that we rather have:
in illam noctem bonam

Hope you don't mind I assumed Russian would help!
Будь здоров.