Why say “tenebrous” when you could say “dark”? The two words are synonyms, after all — the adjective “tenebrous” comes from the Latin for “darkness,” “tenebrae.” But there’s a poetic nuance to certain words, and if you want to imply a mysterious, shadowy, or even otherworldly quality to the darkness, “tenebrous” is the right selection. If we’re talking about somebody turning off the lights when they leave a room, “dark” is fine to use.