Ik vond dit op een ander forum. Misschien wel handig voor de mensen die deze achievements willen hebben:
Note #1: There are more than these 9 skulls, but only these give achievements. All extra skulls will be listed at the bottom.
Note #2: You must start the levels from the beginning or the skull will not appear, do not select a checkpoint.
Note #3: You can only get these on Normal, Heroic or Legendary.
Note #4: You can get these in co-op and the achievement will unlock for everyone playing.
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Iron:
Mission: Sierra 117
Effect: "Death carries a heavy price..." -- Instead of only losing 25 points when you die, you lose 75. You also restart from the beginning instead of any checkpoint.
After Freeing Johnson and killing all the covenent that come in the phantoms, walk to the Pelican that comes to pick you up. Face the cargo bay but instead of hopping in, turn left and follow the path. Once you hit the wall jump up to your right, eventually reaching a walkway. Walk all the way to the end, take a left, then a right, then at the end of this little bit to the right is a dark corner. Turn your light on and you should see the skull. If you cant see it you're probably standing on it.
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Black Eye:
Mission: Crow's Nest
Effect: "Bash your way to better health." -- Your shield will not auto-recharge anymore, you have to kill an enemy with a melee attack.
From the start without moving anything you can see a massive pipe above you. Walk up the stairs then up the next set, turn to your left and jump up on the shelf unit. From there jump to the red cross beam then up onto the pipe, from there just walk to the end and walk over the Black Eye skull to claim it. You don't have to pick this one up it does it automatically.
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Tough Luck:
Mission: Tsavo Highway
Effect: "Your foes always make every saving throw." -- Enemies seem to avoid rockets and grenades easily.
After you have to deactivate a shield that's blocking the road, you will exit onto a bridge and a giant covenant ship flies overhead. You need to get out of your jeep if you are in one and go to the right side of the bridge, then go underneath and you will see bridge supports jutting out into the canyon. Jump onto the one farthest to the left and go to the edge and turn left and you'll see a small jut out of the cliff with the skull on it.
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Catch:
Mission: The Storm
Effect: "Pull pin. Count to three. Throw." -- Every NPC carries two grenades and throws them often.
You can get this by performing a grenade jump off a warthog, or grab a ghost and go back to the ramp from which you first came out, using the ghost to push a big square crate back over to the front of the silo pod that the skull is on top of, then use the box as a place to jump from.
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Fog:
Mission: Floodgate
Effect: "You’ll miss those eyes in the back of your head." -- Your motion sensor disappears.
As you are walking down from the anti-air gun you destroyed in the previous mission, you encounter a ramp (next to a missile launcher). Around this ramp, you hit a checkpoint. At this point, you should also hear a marine yelling, “There! Over There!”. Look up and to the right, directly at the roof of the building next to the missle launcher. There is a single flood form (not to be mistaken with the two other flood forms jumping in front of you) which is holding the skull. Kill him before he jumps, and he will drop the skull down to the ground where you can retrieve it. Too early, and the skull will get stuck on the roof though, so just time it carefully.
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Famine:
Mission: The Ark
Effect: "Trust us. Bring a magazine." -- Dropped weapons will have very little ammo compared to normal.
When you first go into the valley to the right after the wrecked phantom, look to your left where you will see a huge boulder. Use a ghost and get to the side of the boulder that is closest to the bridge overhead. It is easy 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. Simply jump from the top of the ghosts towards the skull and land on the beam.
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Thunderstorm:
Mission: The Covenant
Effect: "Field promotions for everyone!" -- Upgrades enemies to their stronger versions.
After you shut down tower 1 and get access to the hornet, fly to tower 2 (the one the arbiter shut down). While walking up the stairs, go to the middle part that connects both and there should be a stair leading up to a platform where the skull is.
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Tilt:
Mission: Cortana
Effect: "???" -- Enemies have different body parts that may be resistance to certain bullet types.
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Mythic:
Mission: Halo
Effect: "Coverage under the Covenant health plan." -- Gives a 2x score multiplier for all kills and doubles enemies health.
Follow the normal path till you reach a cave, then hug the right wall until it branches off into a small path, at the end of this path is some rubble and in front is the skull.
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De volgende vier skulls krijg je geen achievements voor. Voor de mensen die ze toch willen hebben; zie de spoiler hieronder 
Blind:
Mission: Sierra 117
Effect: "Shoot from the hip." -- Your HUD disappears and you have no crosshair, health bar or anything.
After fighting the first set of enemies, walk downstream while hugging the right wall and eventually you will come to the edge of the cliff. There will be a bit of land jutting out below you with the skull on it.
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Grunt Birthday Party:
Mission: Crow's Nest
Effect: When you kill a grunt, they explode into confetti and children yell, "Yay!"
The second time Cortana interrupts you saying "You have been called upon to serve" you are in the right place, so walk to the end of this path but don't jump off the end. Instead look down and you can see a green arrow, try landing on that ledge, then turn around to see the skull.
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IWHBYD
Mission: The Covenant
Effect: Unlocks hidden dialogue during the campaign.
After killing the prophet truth, kill all the flood in the area but go not go down the elevator at the end. There will be 7 holographic rings that you need to jump through. Jump through them in this order to spawn the skull. Ring 1 is near the elevator with ring 7 being near where you killed Truth.
1 3 4 6 7 5 4 6 5 4 5 3 4
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Cowbell
Mission: The Ark
Effect: Explosive force increased.
After you defeat the scarab, head to the structure in the middle of the area. It's the building with a round front and a sloped back. Grab the Grav-lift item that should be inside. Next proceed through the level and inside. Go past the sleeping grunts and continue past the next set of enemies. You should come to a room with lots of purple glass and a sloped walkway going down. Halfway through the room, at the bend, you'll see some platforms stacked above each other. The skull is at the top level. Go back to the door you entered from and turn around. Look down to see some small lights (dots) on the floor. Place the grav-lift against the right wall just before the fourth light(dot). Now start at the door, run toward the highest platform with the skull, and jump just before the floor starts to slope down and use the grav-lift to propel you up and foward. You should just have enough height to reach the top platform.
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