Spoiler warning: Significant plot details follow.
End of spoiler text.
Usage
Start a spoiler section with {{spoiler|content=text of the spoiler}} which gives:
Spoiler warning: Significant plot details follow.
text of the spoiler
End of spoiler text.
or {{spoiler|the subject of the spoiler|content=sample text}} which gives:
Spoiler warning: Significant plot details about the subject of the spoiler follow.
sample text
End of spoiler text.