It kills everyone if you trhow an grenade if you have defeated THE last boss
it makes all enemies more aggressive :)
it pretty much makes the aliens mad and they try harder to kill you. like the grunt that tries to kill you whe you get the skull.
all of the skulls can be picked up by holding down the "X" button.
It boosts the AI of enemies and allies, meaning that your allies have much greater accuracy, and are less likely to hit you accidentally. Enemies can also hear and react to footsteps and the noise of...