From The Sync Project:
Let’s start by looking at a certain musical arrangement: the tritone. Defined in the simplest possible terms, a tritone is “a musical interval of three whole steps.” For those who can read and understand music, the definition of a tritone can get a lot more detailed than that, so it’s best illustrated with an actual audio sample.
We immediately recognize the tritone from its use to create tension in film, or even to act as a marker between scenes in a play. It’s not that the tritone arrangement is particularly scary per se, it’s that it’s lacking in harmony (it’s dissonant) and is somehow incomplete, throwing the listener off balance and leaving you expecting more (or rather fearing the worst) as it’s played over and over again…
In the Middle Ages the tritone was named “diabolus in musica” (devil in music) and the church in fact banned it from being played. It was seen as the very antithesis of God, which the church believed would be represented by beautiful angelic harmonies rather than incomplete sonic dissonance.
More here.