I am writing a text where this phrase is on a big part. It will be the title, so it's important to me to got it right. Thank you. :)
There's a few ways to translate this. I am ignorant of the nuances, but here's my best understanding based off of minor Googling and Whitaker's Words.
Fructus Tenebrarum = The fruit/benefit/reward of darkness.
Fructus Obscuritatis = The fruit/benefit/reward of blindness/darkness/lack of understanding.
Fructus Crepesculi = The fruit/benefit/reward of twilight/darkness.
If you want to decline this properly in your work (I don't know your intentions) these are the ways you want to decline it.
Subject of sentence: Fructus Tenebrarum/Fructus Obscuritatis/Fructus Crepesculi
Object of sentence: Fructum Tenebrarum/Fructum Obscuritatis/Fructum Crepesculi
Indirect Object: Fructui Tenebrarum/Fructui Obscuritatis/Fructui Crepesculi
Within Prepositional Phrase: Fructu Tenebrarum/Fructu Obscuritatis/Fructu Crepesculi
In addition to Nickimite's fine examples, there is also:
- fructus caliginis "fruits of darkness" (deep darkness, with a vague connotation of being in a dark nebula; gloom, blindness, calamity)
- fructus obscuri "fruits of darkness" (the dark, with a vague connotation of darkness covering the world from one's vision; the hidden, unknown)
I think obscuri is more common than obscuritatis.
Tenebrarum is excellent. It also has the advantage of being grammatically plural (even though it is just "(the) darkness" semantically), so perhaps a suggestion of powers of darkness is most easily made. It is not seldom used with ominous connotations, like the underworld.
Crepesculi is probably uncommon.
Lewis & Short say this:
tenebrae is stronger than obscuritas, and weaker than caligo
As to the form fructus, that can be either plural or singular (one with a final u pronounced long, the other short). Nickimite has given the declension of the singular. Fructus has a connotation of fruit one enjoys, from fruor "to enjoy, profit from". There is also frux "fruit", plural fruges, which is from the same root, but I believe it rather has a connotation of "usefulness".