'Dark' Music

I've recently been tasked with creating an orchestral piece for orchestra which needs to create a foreboding and dark atmosphere and have gotten nowhere with it. I wanted to go for something like this as it builds the tension: 

but I cannot think of any chords or melodies - I usually write happier orchestral pieces and its difficult drastically switching. does anyone have any advice for attempting something like this? any help would be greatly appreciated.

Well, what do you notice about this piece?
Whenever there's a choir, is the sound open, closed, or disjunct?
Is the bottom ostinato tonal, modal, or atonal? Is it driving? If so, when and why does it stop?
What do you notice about the rhythms? Is it in strict time or does it leave it sometimes?
What key are we in? If it's minor, what role does a major chord play in a minor key?