There are 13 skulls in Halo 3, nine of which affect the difficulty of the game, and four of which offer interesting if minor effects. (
You can find more information about the Skull locations in this thread)The nine difficulty skulls are also all tied into achievement points. You can use these skulls to increase the difficulty multiplier on Legendary mode if you're looking to nab the achievements for getting high scores in Campaign Scoring mode. They can also be used to increase the difficulty of Normal or Heroic-level games if you want to get a bit more of a thrill from them, but can't quite bring yourself to play on Legendary.
All of these skills are difficult to find, but most of their positions are noted in the guide, usually under their own header. We'll update the guide as missing locations are pointed out to us. All of the skulls can be found on Normal, Heroic, or Legendary difficulties, but some of them will be quite difficult to reach on anything above Normal due to the increased toughness of enemies around them. You also need to start from the beginning of the level for them to appear; you can't start from a checkpoint or Rally Point if you want the skull to appear.
Each of the skulls below is listed by the level that it's found on first, then by the name of the skull itself. We'll try to link back to the page that has the description or video of where you can find the skull. We'll also describe the effects of the skull when it's activated. To activate a skull, head back to the campaign start screen and hit X to enter the Options menu. You can also turn on Campaign Scoring on this screen.
Golden Achievement Skulls Iron Skull - Sierra 117
When activated, this skull will cause you to return to the beginning of the mission when you die. If you use it in co-op, the death of any player will cause all players to return to the previous checkpoint.
Black Eye - Crow's Nest
If you take any damage to your shield, it simply won't regenerate. In order to cause your shield to regenerate, you have to hit an enemy with a melee attack. Obviously enough, this is only appropriate for levels where you can engage enemies in melee combat regularly. If you spend a lengthy amount of time in a vehicle, you don't want to use this skull.
Tough Luck - Tsavo Highway
With Tough Luck active, enemies will be much better at diving away from your grenades and some of your slower-moving projectile attacks, such as needler and fuel rod gun shots. This will make it much more difficult to get grenade kills, especially on Brutes.
Catch - The Storm
Enemies that throw grenades will throw more of them, and throw them more often. Your Elite and Marine teammates will do the same. Everyone that has grenades will drop more of them, as well, causing some ridiculous explosions during firefights.
Fog - Floodgate
Your motion tracker is disabled when this is active. Not a huge penalty for most players, since most enemies can be located visually, but it's tough when you're fighting invisible Brutes or partaking in vehicle combat.
Famine - The Ark
Every weapon you find off of dead enemies will have a greatly reduced amount of ammo. Some weapons that you find, such as in drop pods or weapon racks, will be full as usual, but this will still have a big impact on your fortunes in combat. Get ready to conserve some ammo!
Thunderstorm - The Covenent
All Covenent forces become tougher versions of themselves. Brutes are more likely to be Captains or Chieftans when encountered, while Grunts will more likely be the toughest gray versions. Flood units will often be the tougher shielded variants.
Tilt - Cortana
The natural resistances of enemies are greatly heightened here. Basically this means that enemies with shields (Brutes, and some Flood units) will only be stripped of their armor if you use a plasma-based weapon on them, while most fleshy enemies will only die to bullets. Brutes will require plasma to strip them of their armor, then you'll need to switch to bullets to finish them off.
Mythic - Halo
All enemies have twice the amount of health. Depending on the difficulty level, this can be anything from an annoyance to a heart attack-inducing source of stress.
Silver Fun Skulls Cowbell Skull - The Ark
This is located deep within the Ark level, near the end of the mission, as you fight your way to the Cartographer. When activated, it will increase the blasts of all explosions in the game: grenades, Needler shots, you name it. They'll all be bigger and better than before.
Blind Skull - Sierra 117
This is one of the first skulls you'll find in the game, as it comes after the first major firefight. When activated, it eliminates your HUD while playing - no radar, no health indicator, no ammo indicator, no nothing.
Grunt Birthday Party Skull - Crow's Nest
This skull unlocks a cheat mode wherein every Grunt will die with an explosion of confetti when you hit it with a headshot.
Unknown Skull - The Covenent?
There is one skull whose location has yet to be revealed as of this writing. It's rumored that unlocking it will let you play the game with a different set of armor on. We'll update the guide as soon as we know more about this skull.
Terminal Locations
There are seven Forerunner terminals scattered throughout the game. Individually they act as backstory exposition; you read them to find out what happened during the last days of the Forerunners. If you find all seven of them, though, you'll earn an achievement. Yay.
Anyway, there are seven in all, but they're only found on three levels. These are short reminder texts on their location; the mission walkthroughs are usually a bit more detailed.
The Ark
The first terminal is near the beginning of the level; you head through a small Forerunner corridor with a room to your left. Head into that room to find the terminal. The second is a bit further on; when you extend the bridge for the Scorpions, turn around and you'll see it. The last one appears underneath a ramped stairwell near the end of the level.
The Covenent
As you make your way into the towers here to shut down the shield, jump across the gaps near the first and third elevators to find two of the terminals. The third terminal is inside the second tower, which you pass by on your Hornet; you'll have to land it and head inside to find the terminal.
Halo
The last terminal is near the beginning of the final level. Keep to the right as you start walking and look for some icy ledges leading upward. Jump into the Forerunner facility to find the terminal.
Linear Mode
