There are so many ways to use processing to make scary sounds that it’s almost too easy. The classic reverse reverb in the original Poltergeist comes to mind. This example is a recording of a conversation with a colleague during a lunch break at a busy sandwich joint. It’s been reversed, pitched down significantly, run through a low pass filter, slowly phased, filter delayed and run through a long reverb. All this processing has diffused the voices into a hellish ambient drone.
Quick and Dirty Ambiance from Hell