Extension:Special404

The Special404 extension provides a simple wiki based 404 special page destination you can configure for your wiki. The body of the 404 page can also be customized using MediaWiki:Special404-body on your wiki.

Web server configuration
Since this is a 404 handler your webserver will need a little configuration to get it working.

Apache
Using an ErrorDocument doesn't seem to work in apache, whatever the destination you set, Apache seems to direct to something that MediaWiki interprets normally instead of as a request to display the special page, so we use a rewrite rule instead. You can place the following in your .htaccess file to make the extension work. Be sure to tweak the path to index.php to match the proper path to yours. Also make sure that this is the last rewrite rule, you do not want any rewrite rules after this one they will likely be overridden. Keep in mind that it's important that you use  (with the proper path to index.php of course) and not something like   as using the latter causes MediaWiki to treat it normally and direct you to the main page instead of displaying the 404 page.

This rewrite rule is not canonical of cause. For example if you use  rule and your MediaWiki is placed at   directory (the most usual case) than the rule will look like this: