What's the definition of tenebrous?

If the clouds are thick and gray and it's been raining all day, you could end up in a tenebrous mood — that is, a dark and gloomy frame of mind. You can even say the weather is tenebrous.

In Joseph Conrad's Heart of Darkness, Marlow ponders on his journey with Kurtz:

It is strange how I accepted this unforeseen partnership, this choice of nightmares forced upon me in the tenebrous land invaded by these mean and greedy phantoms.