“I used a descending chromatic scale throughout the score,” explains Marc Wilkinson, who was director of music at the National Theatre when Haggard approached him to write the score for Blood On Satan’s Claw. “To make it scary, I omitted the perfect fifth, which is the one true consonant in the chromatic scale, and highlighted the diminished fifth, which ever since the Middle Ages in Europe has been known as the Devil’s Interval.” More here. And the BBC has more on the Devil’s Interval or Tritone. “It was recognised to be a problem in music right back to the 9th Century. It is a natural consequence, and so they banned it. They had rules for getting around it.”