spoiler alert

a warning given before the important plot points of a movie, book, game, etc. are revealed

A spoiler alert is a warning that reveals important details or plot twists about a movie, TV show, book, or other media. It is used to alert people that the following information will disclose key elements of the story that could ruin the surprise or enjoyment for those who have not yet experienced it. The purpose of a spoiler alert is to give individuals a chance to avoid or prepare for potentially revealing content before they encounter it.