The sound comes from Nintendo's Super Smash Bros fighting series, first released in 1999, where secret characters challenge the player after certain matches are completed. The flashing new challenger screen became famous in Melee and later games, with the siren version strongly linked to Brawl onward for unlocking fighters. Players grew up dreading and loving that warning, so clips of it spread to YouTube edits, streams, and memes as a universal signal for a surprise opponent entering the scene.