Use a submenu page as parent slug. The admin menu has just two levels, so the imaginary third level will be hidden.

Sample code, tested:

add_action( 'admin_menu', 'wpse_73622_register_hidden_page' );

function wpse_73622_register_hidden_page()
{
    add_submenu_page(
        'options-writing.php',
        'Hidden!',
        'Hidden!',
        'exists',
        'wpse_73622',
        'wpse_73622_render_hidden_page'
    );
    # /wp-admin/admin.php?page=wpse_73622
}

function wpse_73622_render_hidden_page()
{
    echo '&lt;p&gt;hello world&lt;/p&gt;';
}


Add an admin page, but don't show it on the admin menu
When you give the player a rare but powerful item which can only be used once but is never really required to proceed, most players will not use it at all, because they are waiting for the perfect moment. But even when this moment comes, they will still be reluctant to use it, because there might be an even better moment later. So they keep hoarding it for a moment which will never come. 

In the end, they will carry the item around until it is outclassed by other, more readily available resources, or even until the very end of the game. That means that such one-shot items don't provide any gameplay-value at all. They are simply too awesome to use.

What can you do to encourage the player to make use of their one-shot items and not hoard them?
