That which arrives with the night and conceals everything.
Chris Jones at LatinLanguage has a very interesting post about the semantic differences between three Latin words for darkness: obscuritas, tenebrae, and caligo. As usual, he has some excellent examples to illustrate his conclusions. I myself was doubtful if the etymology of tenebrae extends all the way to temetum and temulentus, but indeed, it is linked to Lat. timeo, from there to Sanskrit tamas-darkness, and finally to these more uncommon words.
A good linguist, I am sure, could offer some insight as to how tenebrae evolved from timeo. Alas, it is a better linguist than I.