The Skulls and what they do:Skull 1 - Envy - The Master Chief now has an Active Camouflage device just like the Arbiter's.Skull 2 - Grunt BP - Headshots turn into Plasma Grenade explosions.Skull 3 - Assasins - Most enemies have active camouflage.Skull 4 - ThunderStorm - Causes most enemy units to be their highest rank.Skull 5 - Famine - All dropped weapons have half ammo.Skull 6 - IWHBYD - Unlocks hidden dialogue in campaign mode.Skull 7 - Blind - Your HUD becomes invisible.Skull 8 - Ghost - A.I. charactes will not flinch from attacks.Skull 9 - Black Eye - Your shield doesn't charge normally.Skull 10 - Catch - A.I. will throw more grenades.Skull 11 - Spunknik - The mass of certain objects is severely reduced, making them fly further when smacked with a melee hit or explosion.Skull 12 - Iron - When playing co-op, if either player dies the game restarts you at your last checkpoint.Skull 13 - Mythic - Enemies have more health and shielding devices.Skull 14 - Anger - Enemies and Allies fire their weapons faster and more frequently.Skull 15 - Whuppopotamus - Enemies and Allies know where you are, even if you are using active camouflage. You can't hide.