In order to obtain any of the hidden skulls, you must begin the level at the beginning on the Normal, Heroic, or Legendary difficulty modes. But, in most cases, you will not have to finish the full level after finding the skull, so you can save and quit the mission at that point, if you so choose.

The skulls resemble those that are in the multiplayer game, "Oddball," and can be used as weapons in a similar way. The difference is that the skulls in Halo 3 do not have an information text box when you pick them up. The 13 skulls are hidden and placed out of the way of the normal paths, which means that you must search for them throughout your missions. Otherwise, you must know where the skulls are. The skulls are like Easter eggs on Easter; you must check every nook and crany.

Each skull has a different effect on both you and your enemies. So, you must learn how to use each skull wisely.

You do not need to carry the skull after picking it up if you choose not to, so you can press the 'X' button at the level selection screen to choose whether you want that skull or a different one. By pressing the 'X' button, you will be able to change out the various skulls in order to use them.

This will give you a bonus multiplier for co-op and go towards earning the Guerrilla, Demon, etc. achievements. Do noted that not all of the skulls will earn you an achievement; only nine of the thirteen do.

Gold Skulls will grant you achievements when you first get them, but Silver Skulls will not. You can differentiate them on the selection screen by the color of their background, but in the game, they are identical.

In the Campaign Scoring mode, activating certain gold skulls will grant you a multiplier to the points you receive, depending on the particular skull(s) that you are using.

In Co-op mode, as soon as one player picks the skull up, all players will receive the achievement.

Black Eye Skull: (Gold Skull) At the start of the "Crow's Nest" mission, you want to head strait up to the upper level. Go towards the door with the red light, then turn around. You will want to jump onto the racks, on the red metal light holders. Then jump onto the ventilation tube. The skull will appear at the end.

Blind Skull: (Silver Skull) In the First Stage, when you get to the area where you see the Phantom overhead, jump over the rocks and keep following the path on the right. Once you get to the cliff, there will be a rock over of the side. The skull is on the end of the rock. Note: The Blind skull has to be activated before you start a Campaign map with it, otherwise it will do no good.

Catch Skull: (Gold Skull) In the Storm Mission, you will want to go through the mission from the very start until you go outside again. Once you are outside, look straight across where there is a small, round building, to which the skull is on top. You can either use a warthog as a platform or a grenade-jump in order to get near the skull. Make sure NOT to destroy the wraith near the door or the skull will disappear.

Cowbell Skull: (Silver Skull) In the Ark, you will want to first pick up a grav lift from the small building near where you fought the scarab. Proceed through the level until you reach the second sloping hallway. There you should see some platforms halfway down. The skull is on the top level. You can toss the grav- lift on the right side of the hall so it lands on the fourth little green dot from the door. Then you want to run, jump, and use the grav-lift to propel you to the top. You will reach a check point just as you enter the room so if you miss it just try again.

Famine Skull: (Gold Skull) When you first go into the valley, in the Ark mission, to the right after the wrecked phantom, you want to look to your left where a huge boulder will be. Use a ghost and to the side of the boulder that is closest to the bridge above you. It is easiest to pilot the ghost up the side of the wall using the thrust. To get the skull, pilot 2 ghosts up the wall to the top of the bridge and stack them one on top of another next to the beam where the skull is placed. Then jump from the top of the ghosts towards the skull and land on the beam.

Fog Skull: (Gold Skull) In the Floodgate mission, as you are walking down from the anti-air gun you destroyed in the previous mission, you encounter a ramp. Around this ramp, you will hit a checkpoint. At this point, you will hear a marine yelling, "There! Over There!". Look up and to the right, directly at the roof of the building. There is a single flood form (don't mistake it with the two other flood forms jumping in front of you) which is holding the skull. Kill the man before he jumps, and he will drop the skull down to the ground where you can grab it. If you kill him too early, the skull will get stuck on the roof. You will need to time things perfectly.

Grunt Birthday Party skull: (Silver Skull) In the Crow's Nest, right after the first objective, while en route to the barracks, you will fall down onto a pipe. At the end of the pipe, look over the edge to see a small space a few feet below you. You will need to drop over it, and as quickly as you can, pull back to land under the floor you were previously on. The skull will be at the end.

Iron Skull: (Gold Skull) In the "Sierra 117" mission, where you rescue Sarge, behind the prison cell there is a large ledge. Go to the far right side and jump on the boxes. Then you will want to jump onto the pipes in order to get up on the ledge. From there go to the far end of the ledge, turn two corners, and the skull is at the far end. If you cannot see it you are probably standing on it.

IWHBYD Skull: (Silver Skull) In the Covenant level, you will need to get to the room where you "fight" the Prophet of Truth; let the Arbiter kill him. You need to turn around so that you can kill all the Flood that is in the area as well, as it will make it a lot easier to get the skull. With all obstacles out of the way, you will need to jump through the Halo holograms in a specific order: 4 5 6 5 7 6 5 4 1 2 3 2 7 2 1 3 4 6 7 5 4 6 5 4 5 3 4. When you jump through the last hologram, they all light up in a sequential pattern. The Skull will be at the end right before the energy bridge leading to Truth's corpse.

Mythic Skull: (Gold Skull) Go the normal path through the "Halo" mission until you reach a cave. Get close to the right wall and continue walking until it branches off into a small path. At the end of this path there is some rubble. In the front, the skull will be there..

Thunderstorm Skull: (Gold Skull) In the Covenant, after you shut down tower one and get access to the hornet, you will want to fly to tower two (the one that the Arbiter shut down). While you are walking up the stairs, go to the middle part that connects both. You should find a stair leading up to a platform where you will find the skull.

Tilt Skull: (Gold Skull) In the Cortana mission, when you are in a circular type room with all the flood look for a small structure piece next to two archways, you will want to jump on top if it and up on the rocks to the top left. Then you will want to turn left and jump up again. Turn around in the same spot and jump to the rocks across from where you are standing. After that, you want to follow the the rock sticking out and leading up on top of the original circular room. You will find the skull on a pile of blood.

Tough Luck Skull: (Gold Skull) On the Tsavo Highway, about halfway through the mission, you will come out of a tunnel onto the highway. There you will see a large pipeline to your left. You will want to drop down in between the two and run to the wall in front of you. Then, you will want to follow the wall all the way to where it connects with the cliff on your right. There you will need to turn to the left. There should be a few ledges, you will want to crouch jump from ledge to ledge. On the last ledge, you should find the skull.

Silver Skulls:

Blind- HUD and first person arm disappear

Cowbell- Explosive force is increased

Grunt Birthday Part- When you headshot a grunt, confetti will explode from its head, with the excitement of children.

IWHBYD- Dialouge is changed

Gold Skulls:

Black Eye- Shields are only recharged by melee attack kills, you can achieve overshields by doing this as well.

Catch- Enemies have unlimited grenades

Famine- Ammo picked up or received is greatly reduced.

Fog- No motion sensor

Iron- Restarts mission from last checkpoint when you die in co-op, restarts the whole mission in single player.

Mythic- Enemies have double the health and armor usually given.

Thunderstorm- All enemies will become their highest rank, ex. grunts will become ultra grunts

Tilt- Enemy Armour will become much more difficult to rid of.

Touch Luck- Enemies always berserk, always dive, but never flee.

All pictures can be found at flickr.com