Tenebrous means dark and shadowy. Your big, spooky house with its long, tenebrous passageways and dark corners would be a perfect place to have a Halloween party.
The adjective tenebrous is a perfect way to describe something that's dark and creepy, like a weirdly shadowed room in a castle or a gloomy cave. You can also use tenebrous to talk about things that are figuratively dark, or obscure, like a tenebrous deal made between a city's mayor and the Mafia. The Latin root word, tenebrosus, simply means "darkness."