Halo 3 skulls

Halo 3 Skulls are human skulls hidden in Halo 3's Campaign. Once activated, a skull makes gameplay either more humorous, or more challenging.

Background
The skulls greatly resemble those in the Multiplayer game variant Oddball, and can be used as weapons in much the same way. However, 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 know where they are, to get them. There are thirteen skulls in Halo 3, along with a fourteenth "Unknown Skull", that functions differently from the others.

To unlock a skull, you must start its level from the beginning, on Normal or higher. You can stop playing at any time (provided you resume using the "Resume Solo/Co-op Game" option from the main menu), and you do not need to carry the skull all the way to the end (with the exception of the IWHBYD skull).

The Mythic Skull is the only skull available on Easy, although you will not receive the achievement for it. Also, a very important note is that the skulls are affected by physics. This means that, with the exception of the Famine Skull, skulls can be moved by grenades, explosions, and other forces.

Once a skull is unlocked, you can activate it from the Campaign Lobby. Simply press X while in the Campaign Lobby, and then select a skull with A to activate it. Multiple skulls can be activated at once. The Unknown Skull will not appear on this list.

There are two different types of skulls: Gold Skulls and Silver Skulls.

Gold Skulls will, when unlocked, grant you achievements, each worth 10 Gamerpoints; when activated, they add multipliers to your Campaign Meta-game score. The nine Gold Skulls 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 color, but are identical in-game. The four Silver Skulls affect the physics, dialog, and appearance of gameplay.

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

List of Gold Skulls
For additional help, watch the video at this link.

Iron
Level: Sierra 117

Directions:


 * 1) Battle through to the level normally until you arrive at the point where you must rescue Sergeant Johnson and his team.
 * 2) Snipe 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 blocked line with some pipes. Hop 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 tucked into the corner.

Note: A good strategy for getting this skull is to wait for Hocus's Pelican to show up, and then get the skull.

Icon: A skull marked with a hammer.

Effect: If you die while playing solo, you will respawn at the very beginning of the level - The "Revert to last checkpoint" option is no longer selectable. If any player dies while playing Co-op, all players will respawn to the last checkpoint. This is similar to what would happen when playing Co-op on Legendary in Halo 2, but it is slightly more harsh on the players.

Point Multiplier: 3x

See also: Blind

Black Eye
Level: Crow's Nest''' '''

'''Directions: '''
 * 1) At the beginning of the level, walk to the door leading to the barracks, then turn around.
 * 2) Walk to the green metal rack and jump on top of it.
 * 3) Turn to the right, look up to the red metal beam, and jump on it.
 * 4) To the direct right again, is a much larger blackish pipe. Jump on it.
 * 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.

Effect: Your Energy Shield will not recharge until you perform a melee hit. The Energy Sword and Gravity Hammer's RT attacks count as melee attacks. The shock-wave created by the Gravity Hammer will also recharge your shield. Hijacking also counts as a melee attack.

Point Multiplier: 1.5x

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 CCS-class Battlecruiser 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 on the right, then turn to the left.
 * 5) Look down and you will see a large, black support below you. Jump down onto it. Note that you have to walk and be standing on the part of the pipe that sticks out slightly with the bolts going through it -- otherwise, you'll take too much fall damage and die.
 * 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 on to the cliff, 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 until 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 a three-leafed clover.

Effect: Enemies dodge Grenades, Needles, slow moving projectiles and vehicle assaults with a greater success rate.

Note: Does not affect Pure Forms.

Extra Information: This Skull and Famine seem to increase the likelihood of encountering Kamikaze Grunts.

Point Multiplier: 1.5x

Catch
Level: The Storm

Directions: There are multiple methods which can be used to get onto the building.
 * 1) Battle through to Lakebed A -- the point where you fight an Anti-Air 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. Look for the ramp that leads over to the platform where the normal Wraith is dropped.

Notes:
 * 1) * Possibly the easiest method is to hijack the Anti-Air Wraith and maneuver it so the back (the part with the plasma mortar, or in this case, fuel rod launchers) touches the base of the silo where the ground rises slightly; then, simply get out. The only problem is that this will cause an NPC Hornet to fire upon the standard Wraith, and the Wraith may focus its fire on you, so you will need to work a little quickly. The Anti-Air Wraith should remain still while you jump on the back and onto the top of the silo.
 * 2) * Grenade jump.
 * 3) * Park a Ghost below, then jump from it onto the building. Use a grenade jump if necessary.
 * 4) * 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.
 * 5) * On Co-op, one player can jump onto the other player's head, then onto the building.
 * 6) * Drive the Warthog at the foot of the silo and crouch-jump onto it. Make sure that the Warthog is at the Wraith-side of the silo before attempting the crouch-jump.
 * 7) * Drive a Ghost against the silo, then take your Warthog and drive at it at full speed. The Warthog will end up on top of the Ghost, allowing you do jump up the silo. This may take a few tries.
 * 8) * Pile up a few Ghosts and place a Deployable Cover on them. This should give more than enough room for error once you get on top of it and jump up.
 * 9) * A longer way for those not proficient in grenade jumping or vehicle maneuvering is to keep the Anti-Air Wraith alive and wait for it to destroy one of the Hornets circling overhead. When the Hornet crashes, take a Ghost (or Warthog) and push the Hornet towards the small rise on the dock facing side of the silo. Once there, quickly exit your vehicle and climb up the Hornet until you reach the top of the silo.
 * 10) * While difficult and impractical, you could fly a dumpster over to the skull.
 * 11) Do NOT destroy the standard Wraith, or the skull will disappear. Do NOT physically damage the normal Wraith (though the plasma turret operator can be sniped safely) in any way or the skull will disappear.
 * A Deployable Cover can be obtained by killing the driver of the Anti-Air Wraith.
 * This skull will not appear if a player has beaten the Campaign only on Easy so that they have the Easy Shield in their Service Record. To make it appear, beat the campaign on Normal or a higher difficulty.

Icon: A skull marked with an image of a Plasma Grenade.

Effect: All AI-controlled units, friend or foe, throw grenades more often, where applicable. It even affects scripted grenades. For example, on the level Crow's Nest, on the part where the Warthog is hit by a Plasma Grenade, it will now be hit by two, three, or even four. The Catch Skull has no effect on Flood.

It should be noted that characters will throw an unlimited amount of grenades at their enemies. The skull does not seem to increase the number of Kamikaze Grunts. It has been observed that the Marines will sometimes throw grenades with reckless abandon, similar to their behavior in Halo: Combat Evolved.

Extra Information: This skull is good to have activated in Flood featured levels, since the Flood do not throw grenades but instead drop them when killed.

Point Multiplier: 1.5x

Fog
Level : Floodgate

Directions: At the beginning of the level, walk into the destroyed city of Voi. As soon as you enter the city, there is a building with an ICBM Launcher 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). Wait for a Marine to say over the radio, "All squads, 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 as a Marine replies, "There, over there!".

If you are playing Co-op, station one person 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 as it jumps, with one person using an Assault Rifle shooting straight up, and the other player sniping from the slanted light post. 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.

Another way that takes a lot of skill is to look up at the building and throw a Plasma Grenade at the Combat Form as soon as it appears. The grenade may stick to the Combat Form; if it does, then it will detonate as the Form jumps, killing it and making it drop the skull. Grenade spamming helps avoid the chances of the first grenade blasting the skull back onto a rooftop.

Note: Look up and to the right just before entering the city and continuing to walk into it. You will see it run to the top ledge of the building, but you have a very limited amount of time to shoot the Flood Combat Form. If you shoot while its still on the roof, the skull will probably stay on the roof. 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. 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 once the Marine yells, "We're surrounded, Sergeant!", there will be another checkpoint and you will have to restart the mission (if you missed). However, there is a way to avert this. First, save and quit when you get the first checkpoint. Then, if you miss, do not restart the mission. Instead, you can simply sign out and sign in again and resume the game from the first checkpoint.

Note: This page states that you must hit the Combat Form as it jumps. This has occasionally proved to be untrue. Some people have needed to hit it just before it began to jump for the skull to fall where they could claim it. On higher difficulties, there may be more than one Combat Form on the roof. The one you need to kill is the closest infected human.

Icon: A skull marked with a swirl of fog.

Effect: Your Motion Tracker is disabled completely.

Point Multiplier: 1.5x

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. 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 to 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. - Alternatively you can make it harder on yourself by getting about six 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 three 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 five 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.

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), 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.

Method C: Pick up a Deployable Cover and place it in front of the pillar with the skull and use it as a step to the ledge. It may take a few tries to place it exactly at the perfect spot, as it may slide off and fall off the ledge.

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.

Method E: Deploy a 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.

Method F: Take a Brute Shot, 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.

Method G: Stand on your friend's 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.

Method H: If you saved the Rocket Launcher from the downed Pelican, you can rocket jump by positioning yourself a suitable running distance from the platform with the skull, running at it backward, jumping as you reach the slanted floor and firing a rocket. The blast should be sufficient to propel you to the platform. It may take a few tries to get it without killing yourself, and is inadvisable on any difficulty other than Normal for this reason.

Icon: A skull marked with a smaller skull. The smaller skull seems to have a line between it and some dots (food). This describes the obvious traits of a famine.

Effect: All weapons that are dropped by enemies or are scripted to spawn on the ground (e.g., near a crashed 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.

Extra Information: This Skull and Tough Luck seem to increase Kamikaze Grunts.

Point Multiplier: 2x

Thunderstorm
Level: The Covenant

Directions: After deactivating the first tower, there is a part in the level where you have to fly a Hornet to the third tower. As you hop into the Hornet, the title If You Want It Done Right will be displayed -- if it isn't, then no skull will appear.

On your way to the third tower, you will see the second tower (which was disabled by the Elites). This is also the tower with the Terminal. It looks exactly like the third tower (only without all the land and enemies on it). Fly over to it. You will hear the Arbiter reminding you that the tower is already disabled; this will confirm that you are in the right area. There is a long, flat, semi-enclosed part of the tower that extends out over the ocean. 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.


 * Restarting from Rally Point Alpha allows you to go to the skull's location, but no skull is present.

Icon: A skull marked with a lightning bolt.

Effect: All enemies and allies are promoted to the highest available rank. The number of shielded Flood Elite Combat Forms is also increased.

Point Multiplier: 1.5x

Tilt
Level: Cortana

Directions: The skull is at the top of the large circular room where Cortana is screaming for help. When you enter this room you will fall out of a hole in the ceiling, next to a sitting, dead Elite. 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. 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. Another jump that works is; When you get on the the upper outcrop on the left, facing the other outcrop you are jumping to, move as far as you can to the left. Then, jump toward the other outcrop with the extension of flood material. Walk up the extruded Flood tentacle onto the roof of the area where you will find the Skull in a small pile of blood and bones.


 * Note that underneath the platform you usually walk on are 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 can cause your jumps to fail.


 * You can also jump on the weapon container. You will then jump to a smaller platform covered in Flood biomass, and then onto what appears to be a Flood tentacle. Finally, you will jump to the purple platform.

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 twice as weak to plasma weapons and twice as strong against projectile weapons.

Point Multiplier: 2x

NOTE: Seems to have a similar effect on the player. Immensely increased shield durability (against projectile weapons) observed with this skull active.

Mythic
Level: Halo

Directions: At the beginning of the level, move forward, hugging the right wall and curving slowly left in the cave up ahead. After a short amount of time, 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

NOTE: Seems to have a similar effect on the player. Immensely increased unshielded durability in Sierra 117 as well as Marines and the Arbiter observed with this skull active.

List of Silver Skulls
Silver Skulls provide no score multipliers when turned on..

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 is your first contact with any enemies. Kill all of the enemy troops to get them out of the way. Face the water body (the lake) and move to your right. There is a rock jutting out of the land, and one of the larger rocks extends out over the water. To the right of these rocks there is an old human bunker. The Blind Skull is at the tip of the furthest protruding rock.

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

Effect: You lose your HUD when this skull is turned on. You won’t see your weapons, shield, Motion Tracker, or anything else. This could be used for players recording the output from their console directly to a capture card or recording device of their choice instead of using Theater.

Grunt Birthday Party


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. A Cortana Moment occurs 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 do not 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 with a green arrow just visible in the middle of it. 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. The skull will be located at the end of the passage.

Note: If walking off the edge does not work for you, take a Grav Lift and use it to propel yourself back up to the room underneath the grating after you have jumped down off the edge. Also, the skull may not appear unless you get the other skull earlier in the level.

Icon: A skull marked with a candle.

Effect: If a Grunt is shot in the head with any headshot-capable weapon (such as the Battle Rifle, Beam Rifle, Carbine, Sniper Rifle or Magnum), a popping sound is heard, colourful confetti explodes out of it (the explosion causes minor damage to nearby enemies), and a sound clip of children cheering from the game Viva Piñata can be heard in the background. 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 or injure any single additional Grunt in the nearby vicinity.

Note: To score massive points, one might wish to master the art of "flicking" the thumbstick so as to spread one's fire when firing the Battle Rifle. It is quite a sight when a player finishes off three Grunts with three headshots (yet firing just once) and getting many multipliers for the act.

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, take a Gravity Lift from the small structure in the center (to the right of the entrance up the large slope), then continue on until you pass the room with a lot of explosive Plasma Batteries.

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 Grav Lift to get there. The ideal place to deploy it is about three fourths of the way down the ramp, and a little to the left of the center, while facing up-ramp (facing up-ramp will ensure the Grav Lift doesn't slip farther down). Run down the ramp and jump, such that you reach the peak of your jump while directly over the Grav Lift. You should also be running towards the tall section, aiming towards the middle of the top shelf on the section. If your Grav Lift expires or is destroyed, you can always revert to last save since the room containing the skull is a checkpoint. If you have difficulty getting there, crouch while being Grav-Lifted. This may take a few tries.

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, jump onto it, and 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 (Person A) should drop their Grav Lift before the platforms, use it to go up to the fourth level (one below the skull), and wait for their partner (Person B) to join them. Person B should stand right on the edge of that platform and deploy their Grav Lift while facing straight down. If Person A placed the first one right, then the two Grav Lifts should line up, with one hovering above the other. Person A should then walk backwards off the edge and onto the Grav Lift(s). A will end up being suspended above the platform with the skull, at which point they should just keep holding the left thumbstick forward until they move onto the platform. If they can't move forward, they should keep pushing forward anyway, because when the lower Grav Lift breaks, they will drop onto the platform.

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

Directions: Method E: Another way to get the skull is to push some of the bombs in the room before into the hall right beside the openings in the wall. If you do it correctly, the bombs will look like a raft. Then, drop a gravity lift (it should fall through the bombs) and ride the raft upward. Jump off at the highest point and you will be at 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 fights. It is also recommended to turn on the Thunderstorm skull since Ultra Grunts throw grenades more frequently.

Icon: A skull marked with a cowbell.

Effect: Physics impulses are increased. Explosions and melee attacks launch characters and objects farther, but the damage is not affected.

Unknown Skull (Second Blind Skull)
Level: Cortana

Directions: The Unknown Skull can be found in the Fusion Coil room. First, kill all of the Flood forces in the room. Once the room is free of Flood, search the room for some stacked up Deployable Covers. Make sure you have some grenades (not Spike Grenades or Firebombs) with you. Grab one of the Deployable Covers and climb one of the pillars that extend from the upper floor. Deploy the cover and climb it. Then, jump to the upper floor. If you can't do it with the Deployable Cover, throw a grenade on the end of the pillar and grenade jump -- if you got the Famine Skull by grenade jumping, you should be able to reach the upper floor in 2-3 tries. When you're up, search around the surface and look down. At some point, you should find the Unknown Skull among some bones. It's above the main door.

Icon: This skull is a second Blind skull, so the icon is the same.

Effects: Carrying it throughout the level will make you start the next level with a Spartan Laser and your current secondary weapon.

Note: Obtaining this skull before obtaining the original Blind Skull will make the Blind Skull available on the campaign options screen.

Note: This skull is sometimes referred to as the Cortana Skull, seeing as it has no official name and appears on the level Cortana.

Assassin Skull
The Assassin Skull was found coded inside of the mainmenu.map file in the list of the other 13 skulls.

Possible Levels: Sierra 117, Guardian Forest, Halo

Description/Clues: "Interred where heroes fall."

Effect: "Overactive camo"

Not much is currently known about this skull or its location. Some believe it is hidden under one of the hundreds of pads in Halo, although its true location and ability is still yet unknown.

Mythic Skulls
These skulls can be found in the Mythic Map Pack levels that can give you achievements.

Orbital Skull
Level: Orbital

Icon: A skull marked with the emblem of the Quito Space Tether.

Location: Above the pit between the two pathways, go up until you see the skull on the ledge. You must exit Edit Mode on the ledge beside it, crouch and get it.

Assembly Skull
Level Assembly

Icon: A skull marked with Lekgolo worms.

Location: Near the middle. Between the two pipes connecting the middle to the building. Use Edit Mode in Forge to fly up there.

Sandbox Skull
Level: Sandbox

Icon: A skull marked with a Monitor.

Location: The skull is located between two of the Guardian towers, at the right of the sun -- locating the Skull in Theater first helps. The towers fire upon the player on sight, and are one-hit kills, so to reach the skull, a player has to use Scenery items to protect themselves. The easiest way to obtain the skull is through the following steps:
 * 1) In Forge, Overload the map.
 * 2) Locate the skull and construct a Tube Y-Intersection surrounding the skull. This will serve as your protection against the towers' deadly beam.
 * 3) Place two two-way teleporter nodes; one inside the tube and the other within the map's safe zone.
 * 4) Start a new round and simply walk into the teleporter and obtain the skull.

Heretic Skull
Level: Heretic

Icon: A skull marked with the Mark of Shame.

Location: Standing on the sword platform face the windows and the skull is 90 degrees to the right of the sword under the vents.

Longshore Skull
Level: Longshore

Icon: A skull marked with a harpooned fish.

Location: On the top of a large ventilation shaft beside a "Fronk's" sign. Head for the aircraft carrier & then turn right. Only accessible via Forge.

Citadel Skull
Level: Citadel

Icon: A skull marked with the symbol of the Citadel.

Location: Find the overshield on the blue side. Then look for the wall with the blue triangular shapes on it. There should be a pillar in-between the shapes. At the top of this pillar, to the right in an alcove is the skull. Only accessible via Forge.

Trivia

 * The Mythic Skulls have gold teeth.
 * A Skull's melee power is the same as a regular melee, rather than an instant kill with the Oddball.
 * Halo 3 Skulls also have neural interfaces in the back of them, like those used by John-117, Captain Keyes and Lord Hood.
 * The "I Would Have Been Your Daddy" Skull references to the first chapter of Assault on the Control Room, in Halo: Combat Evolved which shares the same name. The skull name is the same quote from the beginning of the level.
 * On any level with the Arbiter, if you kill the Arbiter in a safe spot, and drop the skull very close to him, he might jump up and grab it and use it as a weapon, going into the fight hitting enemies with the skull as he goes.
 * Halo 3 skulls are generally easier to get than in Halo 2. They can be found on the Normal difficulty, and are often hidden in simple places which require far less skill and time to reach; examples being the Tough Luck, Mythic and Thunderstorm skulls, which only require taking a simple detour rather than a complicated jump challenge.
 * In Co-op, if none of the players have the skull of that corresponding level and someone picks it up all 2-4 players will receive the achievement.

External

 * Bungie.net - Get the most out of skulls
 * Location of all the skulls
 * IWHBYD skull video