Modify main loop in taxonomy archive page

You could hijack the $query just before fetching the posts.

function wpdev_156674_pre_get_posts( $query ) {

    if (
        $query->is_main_query()
        && $query->is_tax( 'region' )
    ) {
        // Manipulate $query here, for instance like so
        $query->set( 'orderby', 'meta_value_num' );
        $query->set( 'meta_key', 'event_date' );
        $query->set( 'order', 'DESC' );
    }
}
add_action( 'pre_get_posts', 'wpdev_156674_pre_get_posts' );

References:

• pre_get_posts action hook
• WP_Query class