Add custom html to last sub-menu item

Ok, so it turns out that by default, $depth in the end_lvl() function starts at 0 when it’s a submenu and increments as the depth gets greater. So we need to test if {$depth} is equal to 0 to apply it only to the first set of submenus:

function end_lvl( &$output, $depth, $args ) {
    if( 0 == $depth ) {
        $output .= '<div>the div that I want to show only once</div>';
    }

    $indent = str_repeat( "\t", $depth );
    $output .= "{$indent}</ul>\n";
}