Bulbapedia:Spoiler warning
Some articles may contain information that may reduce the enjoyment of a narrative work (such as a book, film, or video game) by revealing plot events, twists and endings. If you haven't read or seen the work that the spoiler warning is refering to, you might want to do so before reading the article.
Suggested template
Not all visitors may recognize the site as an encyclopedia, which should strive first to informative, spoilers or not. The article should also contain analysis and background detail not available—or at least, not obvious—in the work being critiqued. Where this is the case, a spoiler notice should be made prominent as a simple courtesy.
The standard way to warn readers of potential spoilers is to insert before the revealing text:
{{spoilers}}
This expands to:
If this general purpose template is not suitable for the particular article you are working on, feel free to custom-design your own warning, but please link back to this page.