One Specific Category To Permalink

Question

I have a Wordpress site that has been up and running for some years using the custom permalink structure of: /%postname%/

This is fine, except I want to launch a special section in which posts go in a directory, for example: /special/%postname%/

What's the proper way to go about this? A custom post type would work, but honestly the template and post type is just an article.

I think there is a way to do this with a rewrite, something like

function append_query_string( $url, $post, $leavename=false ) {
    if ( $category->term_id == 'special' ) {
        $url = add_query_arg( 'special', $url );
    }
    return $url;
}
add_filter( 'post_link', 'append_query_string', 10, 3 ); 

but I'm just starting and could use some pointers. Thanks!