Skull Cheats
Blind Mode
At the beginning of Outskirts jump on top of the light on top of the door with a crouch jump, then jump to the left platform. Turn to your left and go down the long dark corridor and pick up the skull; it will say blind and your screen will flash black for a while. In this mode you can not see your weapon, body, shields, ammo, or radar.
To get rid of this feature use the following method:
Turn off your XBOX or Save and Quit.
Go to Campaign and select level and choose Cairo Station.
Skip the cinematics and then Save and Quit.
Go Outskirt and your hud is back.
Blue Screen of Death
In the multiplayer map Zanzibar, there should be a computer on the upper level on the defense side that opens up the main gate into the base. After you activate this, the screen goes blue much like a system crash in the Windows operating system.
Iron Mode
On the last Master Chief Level, on Legendary Difficulty, when you enter the final Gravity Lift, look up. A skull will pass through you extemely quickly so you must start holding 'X' a bit before you actually pass it to grab hold of it. Once you grab the Iron Skull, "Iron Mode" will be activated.
Envy Mode
On the level Delta Halo (on Legendary Difficulty), progress until you reach the area during the push through the covenant held ruins where you have to clear a landing zone for the Pelican to drop weapons. This area has plasma turrets along its borders. On the left side you will see the outline of a door in a wall, but the door has a solid rock slab in it as well as a rock in front of it. Jump on the rock and then jump to the ledge above the door. Then jump on the ledge to the left, and jump onto the building with the sealed door using grenades. On the top of the building you will find two dancing elites and the Envy Skull. Pick up the envy skull and you will be able to use active camoflage just like the Arbiter.
Assassins Mode
A skull similar to the one that activates blind mode can be found if you start on 'Regret'. After the first lift ride (before the first underwater elevator), grenade jump from the front awning of the lift to the ledge in front of you (the one with two turrets). Grenade jump again on to the next ledge and make your way to the back of the building. While on the Easy difficulty you will only see the skull, but if played on Legendary you will also see 2 invisible Elites carrying swords ignoring you completely.
Grunts Birthday Party Mode
In the level Arbiter on Legendary. Once you get to the first banshee section, fly immediately straight down all the way to the bottom of the map. In one of the cracks where the large cylinders connect to the station. There will be a circle of grunts dancing around a skull. When you pick up the skull; it will say Grunt Birthday Party on the screen. Now when you kill a enemy with a projectile weapon, they will blow up like a plasma grenade was stuck to them. Deactivate this the same as you would deactivate blind mode, described above.