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 referring 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 be informative, whether or not it contains spoilers. Where this is the case, a spoiler notice should be made prominent as a simple courtesy.

To warn readers of potential spoilers, insert the following template before the spoiling text:

{{spoilers}} 

Which expands to:

Spoiler warning: this article may contain major plot or ending details.