Learn French: Le soldat a mentionné que l'horreur était partout dans l'obscurité. - The soldier mentioned that the horror was everywhere in the darkness.

Because the reporting verb a mentionné is in the past, the reported speech must follow the sequence of tenses. If the soldier originally spoke in the present tense, it shifts back to the imparfait. However, if his original words were already in the imparfait describing an ongoing past state, the tense simply remains unchanged as the imparfait.

Does mentionner que ever take the subjunctive?

No. Because mentionner (to mention) is a verb of declaring or reporting a fact, it is followed by the indicative. You would only use the subjunctive for reported clauses following verbs that express doubt, emotion, or necessity.

Are there other common ways to say "darkness" instead of l'obscurité?

Yes. You will often hear le noir used for everyday situations, like being afraid of the dark. For a more dramatic, literary, or eerie feeling, French uses the plural noun les ténèbres (the shadows or pitch darkness). L'obscurité is a standard, versatile noun for the absence of light.

Why is partout placed before dans l'obscurité?

Adverbs of place like partout (everywhere) generally come right after the verb and before prepositional phrases like dans l'obscurité. While putting it at the very end isn't strictly forbidden, placing it immediately after était sounds much more natural in French.