Skulls can only be collected under the Normal, Heroic or Legendary difficulty settings. You must also start from the beginning of the mission; the skulls don't appear if you load from a checkpoint, unless doing so after you die. In Co-op mode, as soon as one player picks the skull up, all players will receive the achievement. Note: Not all skulls earn an achievement; only nine of them do so. To use skulls, press X at the lobby screen to activate a bonus multiplier for co-op and go towards earning the Guerrilla, Demon, etc. achievements.
Black Eye Skull
The description reads "Bash your way to better health." This skull causes your shields to not regenerate until you hit an enemy with a melee attack. Note: You don't have to kill them but simply hit them with a melee attack.
The description reads "Shoot from the hip". This skull causes you to lose your H.U.D., including the shield meter and motion sensor. It must be activated before you start a Campaign map. Also, you will also lose your crosshairs and cannot see what weapon you are wielding.
When active, this skull makes it so that all enemies and allies have two grenades and throw them often. This will also lead to the Covenant sticking themselves.
Using this skull will increase explosive force, just like the Sputnik Skull from Halo 2.
This skull's description reads "Trust us. Bring a magazine." When active, dropped weapons will have less ammo.
This skull's description reads "You'll miss those eyes in the back of your head." When active, will no longer have a motion sensor.
Grunt Birthday Party Skull
When active, this skull makes Grunts explode in confetti when they are hit with a headshot. There will also be some cheering, like a birthday party.
This skull's description reads "But a dog beat him over the fence." This skull activates hidden dialogue from various characters and enemies. If this is the thirteenth skull you have located, it will also unlock the Hayabusa Head. IWHBYD stands for "I Would Have Been Your Daddy" which is a level chapter from the original Halo.
This skull's description reads "Death carries a heavy price...". This skull makes you start from the beginning of the current level when you die. When playing in co-op mode, the death of any player will cause everyone to start from the most recent chapter.
This skull's description reads "Coverage under the Covenant health plan." When active, your enemies have double the usual health points.
This skull's description reads "Field promotions for everyone!" Enemies are stronger than usual.
This skull's description reads "What was once resistant is now immunity." When active, the shields of Brutes and some Flood Combat Forms are nearly impervious. They must be taken down with a plasma weapon.
Tough Luck Skull
This skull's description reads "Your foes always make every saving throw". This skull causes your enemies to be more effective at dodging grenades and other projectiles.