How do I edit the error page ?

This might sound odd, but the error page could be a high traffic page for my site. The main reason for this is that old K2 articles ( event promotions ) are deleted once the event has past. So someone looking for an old event will be sent to the error page.

So based on this, I want to add modules, advertisements, etc. Just basically customize it for my site.

How do I do This?

Thanks

error page is not like everyone template page, it is separate file error.php which you can fin in template directory, you can here module and all template features because this view don't have access to all Joomla settings/features.