MediaWiki extension that allows for spoiler tags that will hide a block of text. The spoiler button's show/hide message can be customized for each set.
To install this extension, add the following lines to the end of the LocalSettings.php file:
//Spoilers
wfLoadExtension('Spoilers');
{{#spoiler:hide=hide_message|spoiler_text}}
{{#spoiler:show=show_message|spoiler_text}}
{{#spoiler:show=show_message|hide=hide_message|spoiler_text}}
{{#spoiler:spoiler_text}}
<spoiler show="show_message" hide="hide_message">
spoiler_text
</spoiler>