ominous (adj) - giving the impression that something bad or unpleasant is going to happen; threatening; inauspicious
This is probably one of those words that sounds cool, but can easily be overused. It's just so easy to go right out and say "ominous" instead of creating an ominous feel with other, less likely words.
The book I'm just starting (another reread) is Joseph Conrad's Heart of Darkness. Talk about ominous. . . Maybe that's why this was the first word that came into my head and hence became this post's topic.