On the level Delta Halo where Cortana says "Their all pouring out of the center", when you are walking into the building where they are coming out of look to your left. Their should be a large cube shaped block blocking a small tunnel. For those of you that know where the envy skull is the block i am talking about is the one that you have to jump on to get the envy skull. Around 5 well placed grenades should move the block just enough for you to fit in to the tunnel. At first you will see nothing but darkness but as you walk forward you will see two glowing beam swords on the floor.