Blind Skull

Like its Halo 2 counterpart, when activated the Blind skull removes all interface elements from the screen. It is a silver skull, and provides no achievements or campaign scoring multipliers.

It is the first skull encountered in the game. It is out in the open, at the end of a rock outcropping overlooking the river to the far right of the pool where the Phantom drops off the first Grunt and Brute reinforcements during the first encounter of the first level, Sierra 117.

Game Guides: 

True Fans Will Not Be Disappointed opines that if you liked Halo and Halo 2, you'll like Halo 3. However, they also like the Flood, which casts aspersion on their judgment:

The enemy A.I. is unrelenting, particularly The Flood. Somehow the developers were able to take one of the most terrifying arch-evil forces and make them even more numerous, gross-looking, and straight up frightening. If there’s ever a jump out of your seat moment, it is during a pitch black encounter with The Flood.