Halo 3 skulls
From Halopedia, the Halo wiki
Summary
A Skull in Halo 3 is an easter egg left in by Bungie to make the game more challenging or interesting, or to unlock potentials that would otherwise remain hidden. There is no information text to wield a skull, as there is with a weapon, but it can be picked up just the same, and its effects range from the useless to the terrifying. In Halo 2, these skulls were hidden very far out of the normal path of the level, sometimes even backwards. There is no reason to suppose that Halo 3 will change this.
In addition, the skulls now have some sort of importance to the Halo 3 Meta-game. They appear to multiply the amount of points you receive in the meta-game, meaning that picking them up will render you more than one benefit.
List of Known Skulls
Black Eye
Level: Crow's Nest[1]
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: Skull marked with one eye.
Effect: Your shield does not charge normally. To charge your shields you must kill something (enemy or ally) with a melee attack (anything but the Energy Sword). You can charge your shields into the Overshield range by doing this.
Catch
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: Skull marked with a stylized drawing of a plasma grenade.
Effect: AI will toss more grenades.
Famine
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: Unclear
Effect: When enemies drop their weapons, they will have considerably less ammo in them.
Fog
Level: Probably Floodgate
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: A skull marked with a swirl of fog
Effect: No motion sensor
Iron
Level: Sierra 117[1]
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: A skull marked with a stylized drawing of a hammer
Effect: When playing co-op, if either player dies the game restarts you at your last checkpoint. On solo, it restarts the mission.[2]
Mythic
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: Unclear
Effect: Enemies have 2x more health and shielding, and are therefore harder to kill.
Thunderstorm
Level: The Covenant[1]
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: A skull marked with a lightning bolt
Effect: Each enemy receives a field promotion and is upgraded to their next available level.
Tilt
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: Unclear
Effect: Enemy resistances are greatly increased. For example, the Brute power armor is normally pretty resistant to human weapons to begin with and fairly fragile against plasma shots. With Tilt turned on, the Brute armor is practically 100% resistant to any human projectiles and can only be taken down by plasma bursts.
Tough Luck
Level: Tsavo Highway[1]
Directions: Unknown
Difficulty: Any ( Reference Bungie Podcast 9/11/2007 )
Achievement Icon: A skull marked with a three-leaf clover
Effect: Enemies always make every saving throw, always berserk, always dive, never flee…
Background
While six of the skulls in Halo 3 have the same names as skulls in Halo 2, their effects may have been changed in some way, or as Bungie sees fit. For example, it is likely that the Mythic Skull still increases the difficulty of killing enemies, but it is not at all clear that it will do it in exactly the same ways as did Halo 2's Mythic Skull. Until Halo 3's release, little more can be said about the matter. Although it could be assumed that the Halo 2 names are actually placeholder names.
Video
<youtube width="425" height="350">x8kiVzzKct8</youtube>
