One way to add some nice density to your chords for some unsettling atmosphere is by adding tones that will creating a m2 interval.
Triads, as you know, are made up of combinations of major and minor thirds. Dissonance mostly occurs in the stacking of two minor thirds or two major thirds (a diminished and augmented triad, respectively).
By either adding a fourth to a major triad, or a second to a minor triad, you create the really dissonant m2 interval:
Not only can it make the sound spooky, but add2 and aad4 chords can create some nice clusters for density purposes.
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