This is not really a secret: How to get to 'Nightmare'.

First, start as usual. Run trough the 'easy' gate. Arriving at the slipgate selector, run to the side. There is a shootable button high up, both sides. Shoot them.

Return to the entry stone. Behind a wall is the slipgate back to the difficulty selector. Shoot it, thats the third switch. Stand next to the entry stone: after a couple of seconds, a lift gets you up to the 'nightmare' selector.