spoiler alert

An indication, placed in a text, that the plot of a drama is about to be exposed.

Spoiler alert: The main character, Mark, succumbs to an illness at the end.

(humorous, often)Indicates that the speaker is about to reveal something unanticipated or unpleasant.

Spoiler alert, these measures have not proved to be very helpful at mitigating the effects of this crisis.