Halo 3 skulls are scattered throughout the game. There are 13 skulls plus 1 undocumented Silver Skull. Nine of the skulls are Gold Skulls, which can affect the difficulty of the game when activated, earn 10 gamer points and gain points multiplier (up to 3 times over the unlocked scores). The other four skulls are Silver Skulls, which only has minor effects and no gamer points.In order to get these Halo 3 skulls, the difficulty setting must be either on Normal, Heroic or Legendary and you must start from the beginning of the mission or level.

Blind Skull
Mission : Sierra 117
Type : Silver
Gamer Points : 0
Effect : Hudless - The Hud and weapons are not displayed.

Iron Skull
Mission : Sierra 117
Type : Gold
Gamer Points : 10
Effect : In Solo campaign mode, if the player is killed, the player will have to restart the mission or rally point. In Co-op mode, if one played is dead, all the players will have to restart the mission or rally point.

Black Eye Skull
Mission : Crow’s Nest
Type : Gold
Gamer Points : 10
Effect : To restore your energy shield, you must melee your enemies.

Grunt Birthday Party Skull
Mission : Crow’s Nest
Type : Silver
Gamer Points : 0
Effect : Once activated, Grunts when hit with a headshot will explode like piñatas.

Tough Luck Skull
Mission : Tsavo Highway
Type : Gold
Gamer Point : 10
Effect : Once activated, all you enemies will be able to dodge more successfully from your grenades, plasma pistol blots and needlers.

Catch Skull
Mission : The Storm
Type : Gold
Gamer Points : 10
Effect : Once activated, the enemies will throw more grenades and more frequently.

FOG Skull
Mission : Floodgate
Type : Gold
Gamer Points : 10
Effect : Once activated, your motion tracker is deactivated.

Famine Skull
Mission :The Ark
Type : Gold
Gamer Points : 10
Effect : Once activated, all weapons dropped by your enemies or allies has 50% less ammo.

Cowbell Skull
Mission : The Ark
Type : Silver
Gamer Points : 0
Effect : Once activated, the explosions will be more powerful.

Thunderstorm Skull
Mission : Covenant
Type : Gold
Gamer Points : 10
Effect : Once activated, each Covenant enemy will receive a rank promotion.

IWHBYD Skull
Mission : Covenant
Type : Silver
Gamer Points : 0
Effect : Once activated, it unlocks hidden dialogues to Halo 3 campaign.

Tilt Skull
Mission : Cortana
Type : Gold
Gamer Points : 10
Effect : Once activated, each Covenant enemy will receive a rank promotion.

Mythic Skull
Mission : Halo
Type : Gold
Gamer Points : 10
Effect : Once activated, all enemies have double the health points.

Second Blind Skull
Mission : Covenant
Type : Silver
Gamer Points : 0
Effect : Undocumented