Place it at the very start of an edit as the warning, then follow it with the Danger Alarm Meme Sound for a second alert before cutting to the FNAF Jumpscare Scream at the reveal, and keep the Inception Horn Sound Effect for the final dramatic moment.
About this sound
This is a slow, wailing emergency siren lasting about six seconds, with a cold mechanical tone that rises and falls like a warning alarm. It feels tense and unsettling, leaving listeners braced for trouble and convinced order has broken down. It comes from The Purge films, where the siren marks the start of the annual purge period, first heard in the 2013 film.