<form name="loginform-custom" id="loginform-custom" action="http://localhost/wp-login.php" method="post">

/**
 * Redirect user on invalid log-in attempts
 */
function login_failed() {
    wp_redirect( home_url('/access-denied') );
    exit;
}
add_action( 'wp_login_failed', 'login_failed' );

function verify_username_password( $user, $username, $password ) {
    if( $username == "" || $password == "" ) {
        wp_redirect( home_url('/access-denied') );
        exit;
    }
}

function restrict_admin_with_redirect() {
    if ( ! current_user_can( 'manage_options' ) && ( ! wp_doing_ajax() ) ) {
        wp_redirect( site_url('/404') );
        exit;
    }
}
add_action( 'admin_init', 'restrict_admin_with_redirect', 1 );