Halo 3 skulls

Skulls are easter eggs in Halo 3 designed to make the game more challenging or interesting. The complete walkthrough to find the skulls in Halo 3.

Background
The skulls greatly resemble those in the Multiplayer game variant "Oddball" and can be used as weapons in much the same way but they lack the information text(i.e. :press RB to pick up) to pick them up. They are usually hidden and out of the way, so one must search for them, or else know where they are, to get them. There are thirteen skulls in Halo 3. 9 Gold ones which unlock Achievements and 4 Silver ones which do not unlock Achievements but are needed to unlock the Hayabusa Spartan Armor for Multiplayer. There is also one 'Unknown Skull' which doesn't unlock anything, but may have a glitch that gives the player a Spartan Laser.

Note: in order to obtain any of the skulls you have to start the level from the beginning and on Normal or higher difficulty. You may, however, use the "Resume Solo/Co-op Game" option from the title menu to resume a level, and still obtain them. You do not have to finish the level nor do you need to carry the skull after first picking it up, so you can simply save and quit, with the exception of the IWHBYD skull. The Mythic skull is also available on Easy (albeit whether or not you will still receive the Achievement is unknown). Also, a VERY important note is that the skulls are affected by physics. This means that a grenade can knock a skull out of its place.

To activate the skulls you merely need to press X in the Campaign Lobby and select any of the skulls you desire to play the game with. The Unknown Skull will not appear on this list.

"Gold Skulls" will grant you achievements, worth 10 Gamerpoints when you first get them, and add multipliers to your campaign meta-game score. There are 9 Gold Skulls. Gold skulls mostly affect the difficulty of game-play, making the game more challenging.

"Silver Skulls" do not unlock achievements or add multipliers to your meta-game score. They are differentiated on the selection screen by their background, but are identical in-game. There are 4 Silver Skulls. Silver skulls affect the physics, dialogue and appearance of the game-play.

Collecting 5, 9, and 13 skulls of either type will unlock the Hayabusa Chest, Shoulders, and Helmet pieces, respectively, for usage in Multiplayer and Matchmaking.

In flood infested areas such as the flood ship in Floodgate or the level Cortana:You can shoot the pods with flood infection forms in them and a skull will pop out but you cant pick them up.

List of Gold Skulls
If you need some help, watch the video at this link.

Iron
Level: Sierra 117

Directions:
 * 1) Battle through to the end of the level until the point where you must rescue Sergeant Johnson and his team.
 * 2) Sniper as many enemies as you can before jumping down, especially on the bridge.
 * 3) Jump down and cross the bridge.
 * 4) Keep to the right of the bridge until you see a block lined with some pipes. Jump onto this block and continue.
 * 5) When you reach the end of this raised block, jump up into the balcony.
 * 6) Follow this balcony to the end, the skull is hidden behind a pillar jutting out from the building.

Icon: A skull marked with a hammer.

Effect: If you die while playing solo, you will restart at the beginning of the level (you cannot revert to the last checkpoint at any time). If any player dies while playing co-operative, you will reset to the last checkpoint. This is similar to what would happen when playing co-operative on Legendary in Halo 2.

Point Multiplier: 3x

See also: The other Sierra 117 Skull, Blind

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Black Eye
Level: Crow's Nest

Directions: Note: There are many other ways in which you are able to jump onto the brown pipe.
 * 1) At the beginning of the level, walk to the door leading to the barracks, then turn around.
 * 2) Walk to the black metal rack and jump onto it.
 * 3) Slowly move to the very right of the metal rack.
 * 4) Jump onto the small black pipes, then onto the large brown pipe in front of you.
 * 5) Walk along the pipe, taking care not to fall off and you will see the skull at the end.

Icon: A skull marked with a smaller skull with a bruise under one eye. Also, the skull’s left eye is marked with a bruise underneath it.

Effect: Your Energy Shield will not recharge until you perform a melee hit. The Energy Sword and Gravity Hammer's R-Trigger attacks count as a melee attack. Hijacking also counts as a melee attack.

In Halo 2, the Black Eye skull could be used to give the player a full 300% overshield, but the recharge effect only worked if you killed an enemy with a melee attack. However, energy sword does not count. In Halo 3, simply hitting an enemy with a melee attack will recharge your shield, regardless of whether the enemy dies or not. However you're no longer able to gain an overshield.

Point Multiplier: 1.5x

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Tough Luck
Level: Tsavo Highway

Directions: Note:This is the first time you come across a highway


 * 1) Keep battling through to the point where you must fight a group of brutes, two of which are using choppers.
 * 2) Kill all the enemies, destroy the shield, then continue through until you reach a long, winding bridge. At this point a Covenant Battle Cruiser should fly overhead.
 * 3) To your left you will see a series of yellow ladders. Go to the very last one. If you receive a regenerator at the top, you will know it is the correct ladder.
 * 4) Jump over or around the large metal bar, then turn to the left.
 * 5) Look down and you will see a large, black support below you. Jump down onto it.
 * 6) Walk along to the end of the support, then turn to the left again.
 * 7) Now you should be able to see the skull on a cliff across a gap. Jump across the gap, and get the skull.

Note: You can get back to the highway. If you jump back onto the pillar you can carefully walk around the ledge untill you reach the grassed area, jump off and up the hill to your right and you will be back on the highway to continue on your way! Alternatively, the player can simply return to the previous checkpoint - though you don't keep the skull, the achievement remains.

Icon: A skull marked with three clovers.

Effect: Enemies dodge grenades, needles, and vehicle assaults with a far greater success rate. However, plasma and spike grenade sticks are still possible at close range, and from behind.

Note: Does not affect Hunters or Ranged Pure Forms.

Point Multiplier: 1.5x

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Catch
Level: The Storm

Directions:


 * 1) Battle through to the point where you fight an AA wraith, a handful of Ghosts, and a standard Wraith.
 * 2) You should see a silo like building. (See image) On top of this is the skull.
 * 3) There are multiple methods which can be used to get onto the building.
 * 4) * Grenade jump.
 * 5) * Park a vehicle below, then jump from it onto the building.
 * 6) * Take a deployable cover, then use it between the red wall and the building. Move out of the way and you can use the cover as a stepping stone.
 * 7) * On Co-op mode, one player can jump onto the other's head, then onto the building.
 * 8) * Use combinations of vehicles, grenade jumps, and other players.
 * 9) * Drive a Ghost to the foot of the silo, then stick it with a plasma grenade. The explosion of the grenade, coupled with the explosion of the Ghost, gives more than enough height.

Notes: Icon: A skull marked with an image of a plasma grenade.
 * If you destroy the standard Wraith, the skull will vanish.
 * A deployable cover can be obtained by killing the driver of the AA Wraith.
 * If you die once, the skull will not respawn again. Clicking the "Restart Mission" button to refresh the spawn does not help either. The only way is to "Delete" the saved game by changing the difficulty of the level, and changing back again or change the campaign mission to a different one, and back to "The Storm" again the refresh the Skull spawn point.
 * The campaign must be beaten on Normal difficulty or higher prior to acquiring the skull or it will not appear.
 * This may be one of the easiest skulls to work with. Once activated you have to dodge a lot more grenades, as the effect states, but you also get WAY more grenades to work with. The only difficulty with this skull is if you are driving a vehicle and you are charging into an area packed with grunts you have to really careful not to get stuck because they have been observed throwing up to four plasma grenades in a row.
 * The skull will not be there when you first look for it,it's best to get the skull before the standard wraith appears as fast as you can.

Effect: All AI-controlled units, friend or foe, throw grenades more often, where applicable. The Catch skull has no effect on Flood, as they don't throw grenades. It should be noted that they will throw a unlimited amount of grenades at their enemies. Grunts have been seen throwing more than 4 in a row. The skull does not seem to increase the number of Kamikaze Grunts, however.

Point Multiplier: 1.5x

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Fog
Level : Floodgate

Directions: At the beginning of the level, you will walk into the destroyed city of Voi. As soon as you enter the city, there is a the first main building with a missile installation on the right. Move to the middle of the road that leads in and left. Take out a weapon with a scope (preferably a battle rifle) and look up and to the right (to the top of the right hand building). Possibly wait for the sergeant to say "squad, report!" You will see a single flood combat form on the roof of the building to the right. It will jump across the street above you and a marine to starts yelling "there, over there". If you are playing co-op, station one man in the middle of the road between the burning building and the building the creature lands on (Check where he goes before reverting to last save.) Kill the flood just before/as it jumps, one guy using an assault rifle shooting straight up, and one guy sniping from the slanted light post with a battle rifle. Hopefully, the skull will fall off the right hand building so you can pick it up. It may come to rest near the bottom of the left-hand building. This should take several tries. If you miss the flood or the skull is stuck on the building, just restart at the last save.

Note: Look up and to the right just BEFORE entering the city continuing to walk into it. You will see them run to the top ledge of the building, but you have a VERY limited amount of time to shoot the Flood creature. But the first time, if you're unable to shoot them, you will at least have a good idea of where to be looking the second time, and just revert to saved. Tip: you only need one bullet, no matter from what gun to bring the skull down. Since the level is very dark, you may miss the skull being dropped, so search around to find it.

Note: You will have to revert to last save quickly if you miss because after the marine starts yelling "we're surrounded sergeant!" there will be another checkpoint and you will have to restart the mission (if you missed).

Note: If you are a nervous player you may wish to disable this skull while playing levels that have booster back brutes which can go up into the air, fly above you, and then kill their boosters dropping themselves right next to you.

Icon: A skull marked with a swirl of fog.

Effect: Your motion sensor is no longer active.

Point Multiplier: 1.5x

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Famine
Level: The Ark

Directions: Method A At a point in the level, you will be directed into a fork in the road. Going straight will lead you into a giant bowl-like valley, and going to the right will lead you through a broad sandy road about 40 feet wide. To the left is the bowl with two roads cut into the rock face. Go to the right and drive directly ahead until you hit the rock wall. When you get underneath the platform structure, you will notice some climbable rocks on the right that will lead you to the platform itself. You will notice several beam-like supports alongside the platform. Go the very last one, at the end of the platform. If you jump up you can see the skull at the end. Align yourself right up to the support. You need to get to the crevice of it, sticking out and away from the platform; the skull can be seen in it. Stick a Plasma Grenade right where the beam-like support and incline meet(on the bottom edge of the incline). Back up, run and jump, aiming a little higher than the skull. You should be just over and a little forward of the grenade when it explodes giving you enough of a boost to reach the skull. (Note: you don't need to back up excessively. The process from the moment you throw the grenade to when you start running towards it should take less than 2 seconds). - Alternatively you can make it harder on yourself by getting about 6 feet away from the incline, and throw a plasma grenade to the top of the incline. It will stay there, because the incline rises about 3 inches before it goes downward. As you throw, begin a jump toward the crevice of the support. If you timed it right, you should be about 5 feet past the grenade when it explodes, which should give you the boost you need to get onto the support and get the skull. The timing might take a few tries before you get it right. -It is best to do Method A on Normal Difficulty as the grenade blast will take your shields to the brink. Don't be lazy, take care of the two ghosts first.

Directions: Method B A slightly easier way to get the skull is to use a Brute Chopper. Save one from an earlier battle, and then drive it to the part that's underneath some kind of roof (this is where you're going anyways) drive out from underneath it, and you should be making a right turn. To the right, on the rock face, there is a lone boulder on the ground. Drive slightly past it, then turn around, aiming at the bridge, and drive up the steep face. Eventually, you will be at a small gap between the bridge and the rock. on the left "edge" of the rock, you can see a small upwards incline. Using this as a jump, boost off of it and try to land on the bridge. This will probably take a few tries. Once you're on, turn left and drive to the last "pillar" (which contains the skull) hugging the pillar, drive the Chopper as close to the edge as possible. Then just jump on top of the Chopper, and jump into the pillar. Getting the Chopper just right and getting into the pillar takes a few tries, so don't be discouraged if you miss and take a while.

Directions: Method C When killing Brutes in the level pick up a deployable cover and drop it in front of the pillar with the skull and use it as a step to the ledge.

Directions: Method D Play the level on Co-Op with a friend. Both of you should climb up on to the platform. Have your friend stand as low on the incline near the pillar as he can with his back against the pillar. Then your friend should kill you, and when you respawn you will land right on his head, use him to jump up to the platform, grab the skull, and you both get the achievement.

Directions: Method E Yes, another method but this is for those who can't grenade jump for their lives. Quite simple really, just take a bubble shield and at least one fragmentation grenade and go to the ledge. Deploy the bubble shield so that the edge of it is near the edge of the structure. The bubble shield should keep the grenade you throw inside it so right after you throw, jump, and the grenade should give you a sufficient boost.

Directions: Method F One more method. This is for people who know how to grenade jump but by some misfortune use their grenades. Take a brute shot, they're all over the place on that level, and aim it downward but still on the building Take a leap and as you jump fire downward and you will be able to launch yourself to the skull.

Directions: Method G One More method. Was found by Accident in CO-OP so it's still experimental. stand on your friends head. Then have them grenade jump. At the highest point of their jump, jump over to the ledge. If performed correctly, you should land on the platform.

Directions: Method H On co-op, if you can't find any brute shots, take a fuel rod gun dropped by the grunt heavies on the far side of the big bowl (from the entrance) near the platform, and climb up onto the other platform with the skull on (with your co-op partner) Go to the support on which the skull is and stand on their head and jump and fire down (onto your partner) at the same time and you should easily get onto the support, which you can walk along to get the skull. Note that this does not work as well on solo because the fuel rods just bounce off the slanting floor.

Directions: Method I An easier way is to get a deployable cover dropped by some Brutes near the beginning of the level, deploy it near the wall, and jump on it, then jump from that to the ledge, and crouch down to get the skull.

Icon: A skull marked with a smaller skull.

Effect: All weapons that are dropped by enemies or are scripted to spawn on the ground (e.g., near a downed Pelican or group of dead bodies) have halved or reduced ammunition or charge. However, weapons found in cases still have the regular amount of ammo.

Point Multiplier: 2x

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Thunderstorm
Level: The Covenant

Directions: After taking down the first tower there is a part in the level where there are two hornets and you are expected to take one and get to the third tower to take it down. The screen will briefly show "If you want it done right" as you hop into the Hornet. If it does not say "If you want it done right", then no skull will be present. After you get in, start to fly to the third tower. On your way there, you will see the second tower (which was disabled by the Elites). It looks exactly like the third tower (only without all the land and enemies on it). Fly over to it. You will hear an Elite reminding you that the tower is already disabled. This will confirm you are in the right area. There is a long flat part of the tower that extends out over the ocean (semi-enclosed). There is a very tall ramp that leads up to it. At the end of the extension you will find the Thunderstorm skull. You must get out of your Hornet or Banshee, or the skull will not appear.

You must start from the beginning of this level, disabling tower one then moving outside in order to pick up this skull.


 * Restarting from Rally Point Alpha allows you to go to the skull point, but no skull is present
 * Starting from Rally Point Beta (with a Tank and Warthog) leads to a fight with two Scarabs, and is beyond the skull pickup point

Icon: A skull marked with a lightning bolt.

Effect: All enemies (and Sangheili allies) are promoted to the next available rank. The number of shielded Flood combat forms is also increased. Also it seems that the Chieftain on Sierra 117 sometimes become a War Chieftain. Elites that would have been Majors do not become Ultras, however, and stay the same rank. Basically, field promotions for everyone!

Point Multiplier: 1.5x

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Tilt
Level: Cortana

Directions: The skull is at the top of the large circular room where Cortana asks your name, and says it’s very nice to meet you and that she likes to play games too (your screen and movement won’t change for this one). To the left of the room will be some small, somewhat fungal, growths on the walls. Near this is a rounded pillar. Stand on the pillar and jump to the thin growth outcrops to the left. On many of them you will hit your head on an obstruction above if you are not careful - spoiling the jump; try backing up slightly first. After the first two to the left, turn around and look away from the rock wall - then left ninety degrees for another one with a longish extrusion of Flood material. This one is difficult as your head will hit the wall with jumps from many of the obvious positions. One jump that works is at the tip of this jump, back about 2 feet from the edge, and jumping slightly left (approx one foot) of the tip of the target jump. Walk up the extruded Flood tentacle onto the roof of the are where you will find the Skull.


 * Note that underneath the platform you usually walk on are a two to three deployable covers near the terminal with Cortana's message. These can be placed on the pillar and the ledge to make the jumps easier and help you avoid things that you cause your jumps to fail.

Icon: A skull marked with a smaller skull with a hole in its head.

Effect: All enemy weaknesses and resistances double. Example: Brute armor is weaker to plasma weapons and stronger against projectile weapons.

Point Multiplier: 2x

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Mythic
Level: Halo

Directions: Move forwards, hugging the right wall and curving slowly left. After about 15 seconds you will see a tunnel opening to the right. Move through this, but still hug the right wall. Part way along an opening (cave) on the right will open up. Move inside turning your flashlight on (up on the D-pad of the controller). Soon you will reach a dead end, and the skull is on the ground, among the rocks.

Icon: A skull marked with a circle with a smaller circle in it

Effect: All of the Covenant you encounter have twice the amount of health.

Point Multiplier: 2x

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Blind
Level: Sierra 117

Directions: In the beginning of the level, you and your troops will come across a waterfall with a Phantom dropping off troops. This your first contact will any enemies. Kill all of the enemy troops to get them out of the way. Face the water body (lake) and move to the right of the area towards the lake. There is a rock jutting out of the land, and one of the larger rocks extends into the water. The blind skull is at the tip of the rock. There is also a second blind skull on the level Cortana but no bonus is gained for capturing both skulls. Though that is a fun challenge on Legendary.

Icon: A skull marked with an eye with a crossed out pupil. Also, the skull’s eyes are shaded white.

Effect: Your HUD and weapon are no longer visible.

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Grunt Birthday Party
http://images.wikia.com/halo/images/b/be/1213124784_Grunt_grun_gru_gr_g.jpg

Level: Crow's Nest

Directions: On your way to the barracks (after you view the bomb, and after the flying Drones), you will come across a part in the level where you drop down a hole with fallen rocks at the end. Cortana sends you a message ("you'll be called upon to serve") when you land. Once the message is over, you will find yourself in a wide corridor with open pipes on either side, with Drones flying across every now and then. If you walk to the end of the corridor, there is another large drop (purple pipe with a rotating fan), but DON'T GO DOWN. If you go to the very edge and look down the drop, you will notice a small ledge protruding out a little bit. If you approach the edge very slowly and walk backwards very quickly as soon as you fall (or vice-versa), you will land on the ledge. Behind you (or in front of you if you backed towards the drop and then ran forwards) is the skull.

Note: If walking off the edge does not work for you, take a gravity lift and use it to propel yourself back up to the room underneath the grating after you have jumped down off the edge. Sometimes the skull is not there, this is because you have to kill all of the drones.

Icon: A skull marked with a candle.

Effect: If a Grunt is shot in the head with any semi-automatic weapon (including the Battle Rifle) confetti explodes out of it (the explosion causes minor damage to nearby enemies), and a sound clip of children cheering from the game Viva Pinata can be heard in the background. With good aim, you can also cause the explosion with a Magnum. When the grunt's head explodes, their mask will usually fall off

Note: If you are playing with any of the skulls active you should have this skull active. It does not increase the difficulty of the game, it actually makes it easier. When you blow a grunt's head off the explosion slightly damages nearby enemies. Groups of three or more grunts, if you kill them quickly, within 3 seconds or so with headshots, will most likely kill any single additional grunt in the nearby vicinity.

Note: You can set off large chain reactions if you activate Cowbell and catch in conjunction. First kill Grunts by shooting them in their chests to make them drop their plasma grenades. Do this repeatively until the plasma grenades build up. Next shoot one in the head to blow up their head. This will set off all the built up grenades. Sit back. Watch. Enjoy.

Cowbell
Level: The Ark Directions: Method A: You must start this level from the beginning - starting from any other point will fail. After defeating the Scarab, grab a gravity lift from the small structure in the center (to the right of the entranceway up the large slope), then continue on until you pass the room with a lot of explosive plasma containers. In the hallway after this room, there is a tall section with openings above the turning point of the hall, and the skull is in the top one of the highest of these. Use the gravity lift to get there. The ideal place to deploy it is about 3/4's down-ramp and a little to the left of the middle, facing up-ramp(facing up-ramp will ensure the gravity lift doesn't slip farther down); you should be running down-ramp and jump so your jump-apex is above the gravity lift. You should also be running towards the tall section, aiming towards the middle of the top shelf on the section. If your gravity lift is destroyed, you can always revert to last save since the room containing the skull is a checkpoint. Also if you have difficulty getting there crouch while being grav-lifted. If you don't get the skull at first then keep trying, you will get it eventually.

- Alternatively you can place at the bottom of the ramp and use a Brute Shot jump for extra height.

Directions: Method B: If you are having too much trouble placing the grav lift, it is possible to drive a Brute Chopper into the building (there's one inside the small structure where you battle the Scarab). You can then drive the Chopper to the bottom of the ramp, You can then easily walk right into the opening with the skull.

Directions: Method C: If you are playing on Co-Op, have two of the players grab a Grav lift. One drops his or her Grav lift before the platforms and goes up to the fourth level (One below the skull) and waits for his or her partner to join them. The person still holding their Grav lift stands right on the edge of that platform a drops their Grav lift facing straight down. If you have placed the first one right then they should be inline. stand on top of it, look down, and deploy the Grav-lift. One person walks of the edge onto the Grav lift backwards. You should then be suspendid above the platform. Just keep pushing forward with the left sitck and you should move onto the platform. If not, keep pushong forward with the left stick and when the Grav lift on the floor breaks then you will drop onto the platform.

Directions: Method D: It is possible to get the cowbell without using a friend, brute shot, or Grav-lift. All you need is some jumping skills. You can jump from the ramp to the first platform and then to each platform above.

Directions: Method E: It is possible to get the Cowbell by using a combination of both an explosive crate and a stick grenade. Push the crate down the stairs to the correct location (as above), then stick the grenade on top. Wait approximately one second, then jump (as the grenade explodes, setting off the crate). This will send you sufficiently high to make it to the 4th level. Do not hold down the right "take" button as you do so, as this may negate picking up the skull.

Note: It is suggested to always couple this skull with the Grunt Birthday Party skull for maximum damage when encountering large numbers of Grunts.

Note: Another possible use for this skull would be to couple this skull with Catch (so that grenade explosions are deadlier and occur far more frequently), creating a greater sense of danger in large firefights.

Icon: A skull marked with a cowbell.

Effect: The force of explosions is increased immensely.

Note: While this makes life more dangerous for you it also makes it a bit easier. Your grenades will be more destructive, obviously, but also the force of the explosions from the grunts heads exploding (with the Grunt Birthday Party skull active) will increase as well.

Note: This skull should be used in conjunction with the Grunt Birthday Party Skull (or the Catch Skull if you want a challenge.)

Note: This skull weakens your melee attacks.

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IWHBYD (I Would Have Been Your Daddy)
Main article: Halo 3: IWHBYD (I Would Have Been Your Daddy) Skull

Level: The Covenant

Directions: In order to get the IWHBYD skull, you must complete the entire mission (eg. do not use Rally Point Alpha) until you reach the final battle scene, where the Prophet of Truth is planning on lighting all seven rings. Once you reach the bridge, battle your way across with the Arbiter and Flood (making sure not to jump through any rings), neutralize all Covenant, watch/skip the cinematic, and kill all the flood (this is optional, but makes the procedure much easier). Once the bridge is devoid of enemies, you will have to literally jump through the 7 rings in a complex order. Once you have completed this, the IWHBYD skull will appear at the beginning of the light bridge leading to Truth's podium.

Each number corresponds to the ring you must jump through. The ring at the elevator is 1. The ring next to Truth is 7. The ring with red on it is 4.

Sequence orders:
 * Quick sequence(easy, doesn't always work) 4,6,5,4,5,3,4.
 * Longer sequence (sure-fire way) 1,3,4,6,7,5,4,6,5,4,5,3.
 * Definite sequence (DEFINITE WAY TO DO IT) 1,3,4,6,7,5,4,6,5,4,5,3,4,5,4,5,3,4.

Once you have completed the sequence, the skull will appear at the beginning of the light bridge leading to Truth's podium. If you are not sure you got it then look at the rings when you complete the sequence, they will be blinking in sequence. The Halo theme will play as you get the skull, and the rings will stay on brightly and sometimes flash. You must then finish the level, or the skull will not be saved to your profile and you will have to get it again (eg: jump down the blue hole at the far end as per the normal end to the level).

The skull will not spawn if you have jumped through any of the rings before you reach the Prophet Truth. If you do by some means jump through a ring before Truth is dead, then simply restart from the last save.

Icon: A skull marked with a double helix (DNA Symbol).

Effect: Unlocks new, often humorous, dialogs for the AI. Such as "THIS IS SPARTA!" for some marines or "Pie is good" said by some Brutes. Sometimes a Brute may even say "He was my lover!" when you kill a nearby Covenant soldier, and Grunts may say "Stay close to me, Nipple-mate!" to another Grunt when they are suspicious that enemies may be nearby, or "Thank you for choosing Air Blue-Ball" as they go Kamikaze. Also, after winning a firefight, a marine will sometimes say "Yeah, they were all like, 'We're gonna take over Earth', and we were all like 'Yeah? Eat these grenades!'" And, if you stare at a female marine's chest with the skull on, she will say "Keep the eyes on the green zone, soldier."

Note: Be sure to actually jump through the rings, not just go through them

Note: This skull does not affect the difficulty of the game. It just makes the game a bit more fun.

Unknown Skulls
There is a 5th silver skull, that does not appear in the Campaign Options screen. It is on the level Cortana, in the reactor room after retrieving the AI. It can be obtained on Co-op with a Deployable Cover. Deploy it near the edge of the roof, and get Player Two to jump on it and Player One on top of Player Two. Player One should be able to jump on the edge of the roof and get the skull. Please note that this skull has no affiliation with the 13 required skull list, and if carried for the rest of the level, Player One will start with a Spartan Laser on the next level. It may also have an effect on health, shields or the amount of damage enemies can do. Caution: this may only work with certain skull combinations and/or other factors in gameplay. This skull is a duplicate blind skull.

Also, there is another unknown skull in the beginning of the level Sierra 117 in the very first Covenant Loyalist encounter. It can be found opposite from where the second Phantom deployed units. It is on a rock on the edge in the corner.

Trivia

 * A skull's melee power is the same as firing a shot gun.

External Link
Bungie.net - Get the most out of skulls