trombone
n.n. a musical instrument with a slide that you move to change the sound. In movies and TV, it is often used to signal that a character is about to do something very bad or violent.
n. a brass instrument with a telescoping slide used to alter the pitch. In cinematic contexts, it is frequently associated with the 'trombone effect' — a sudden, ominous sound used to signal a character's impending death or a violent act.
The trombone played a loud, scary sound when the villain appeared.
As soon as the hero walked into the dark alley, the trombone started playing to signal the audience that something bad was about to happen.
The director used the trombone to create a sense of foreboding, a technique that has become a staple of the horror genre to cue the audience's emotional response to the protagonist's fate.
Borrowed from Italian trombone, from tromba (“trumpet”) + -one (augmentative), literally “large trumpet”. The telecommunications sense alludes to the shape of the musical instrument.