Eternal darkness would be
tenebrae aeternae.
Latin can be challenging to translate. As in French and Spanish, the adjectives have endings that have to agree with the gender of the nouns they modify, but in Latin, the nouns also have case endings that change according to what role the noun plays in the sentence, so the adjectives have to agree with both the gender and the case. Then there are the verbs, which have five regular conjugations, compared to the three you find in French and Spanish. The rules of syntax are also looser than in English. All this tends to muddle computerized translations, so your most reliable option is to just learn Latin. (Easier said than done, I know.)
Having said that, the online
Lewis and Short dictionary is a reliable tool for Classical Latin-to-English translations. You can also do English-to-Latin searches on
this page.
This glossary is handy when you're dealing with medieval Latin, and
this lexicon from the Vatican can be helpful when you're trying to figure out how to translate more modern words like nylon, umbrella or laser.
Google also recently introduced a
Latin translator. It's surprisingly good, for an automatic translator, but sometimes hilariously wrong.