Put it at the end of your build-up, immediately before the reveal, then follow the reveal with a Scream Meme for the shock payoff and an Emotional Damage sound if the twist lands badly for the person on screen.
About this sound
This is a short spoken warning, lasting about 2.7 seconds, in which a voice delivers the line 'you have no idea what's coming' with slow, deliberate menace. It creates a tense, anticipatory feeling, like the calm moment before a surprise or twist is revealed, and works best when the listener already senses that something big is about to happen.