spoiler alert

Short definition from Wiktionary

Noun

1. 1
   Used other than figuratively or idiomatically: see spoiler, alert: a warning that plot elements in a drama will be exposed.

Interjection

1. 1
   An indication, placed in a text, that the plot of a drama is about to be exposed.
2. 2
   Indicates that the speaker is about to reveal something unanticipated or unpleasant.