1. Blind Skull: Level: Sierra 117

Jump down the water fall (Where the two phantoms come in bound) and head right, past the dead tree. You'll see it on the ledge below. - Effect: No HUD.

2. Iron Skull: Level: Sierra 117

At the very end of the first level, after you've rescued Johnson, turn left and jump up to the ledge (With the "No Fishing/No Swiming" sign) instead of boarding the Pelican. Walk to the end and jump up to the next ledge. Walk all the way around the building and you'll find the skull tucked into the corner. - Effect: Start from the begining of the level when you die. (No Checkpoints).

3. Black-Eye Skull: Level: Crow's Nest

At the start in the UNSC control room, go up the steps and all the way to the back. Jump on the racks and up to thebig pipe. Walk to the end for the skull. - Effect: Only melee attacks can regenerate shields.

4. Grunt Birthday Party Skull: Level: Crow's Nest

At the ventilation shaft (Where the drones fly in and out) just before you jump down to meet the Arbiter, look below the edge and you will see a small ledge with a green arrow sign pointing to it. - Effect: Killing Grunts with headshots will produce a humorous effect.

5. Tough Luck Skull: Level: Tsavo Highway

After you exit through the tunnle (past the blue barrier), you will see a giant pipe to the left. Toward the end of the pipe you will see support beams extending out over the gap. Jump onto the first beam (The one closest to the left wall), then go to the end and you'll see the skull sitting on the ledge to the left. - Effect: Enemies are more aggressive and are able to "Make Saving Throws" (Dodge projectiles better).

6. Catch Skull: Level: The Storm

When you first encounter the Wraith tanks, keep one of them alive; the skull will dissappear if both of them are eliminated. Drive a vehicle right up to the circular structurenear the red Wraith. Stand on top of the vehicle and use a grenade jumpto get up to the roof (Throw down a grenade, jump and use the explosion to give yourself a little boost). It'll take some practice. A Grav. Lift will work here too, but they're hard to find. - Effect: Enemies throw many grenades.

7. Fog Skull: Level: Floodgate

At the start of the level, when your walking down the hill to ward the burning buildings, keep your eye on the first building to the right (Behind the missle launcher). As soon as you hear the soldiers talk on your radio, a Flood will appear holding the skull on the rooftop. Shoot him quickly before he jumps away. The Skull will fall to the ground so you can retrieve it.

8. Famine Skull: Level :The Ark

After clearing the off the deck, Johnson will appear in a Pelican and ask you to follow him. As you make your way down the Canyon, you will see a large bridge/deck coming out of the rock on the left (Just where the ghosts come attacking). Use the rock face around the corner on the right to to jump up and hop over to the extending ramp. Walk to the last beam and you will see the skull sitting just inside of the loop of the extending structure. You will need to do a grenade jump, deploy a shield and use it as a platform or use a Grav. Lift to make it up to the skull. - Effect: Weapons dropped by enemies will have 50% less ammo.

9. Cowbell Skull: Level: The Ark

Pick up the Grav. Lift inside the rounded bunker next to where the Scarab battle occurs. Hang on to it as you make your way up to meet the Arbiter. After clearing out a few rooms, the Arbiter and the team will hold position at the entrance to a ramp that leads downstairs. In the middle of this hall you will see several openings that are stacked all the way up to the ceiling. The skull is in the top opening. Use the Grav. Lift to boost up to it. - Effect: Bigger explosions.

10. Thunderstorm Skull: Level: The Covenant

When you pilot the Hornet, land on the second tower (The one the Arbiter cleared) before heading to the battle by the third tower. You will find the Skull at the top of the ramp. - Effect: All enemies go up one rank.

11. IWHBYD ("I Would Have Been Your Daddy") Skull: Level: The Covenant

This Skull is located in the hall with the seven blue Ring Holograms (Spread out in three sections). You'll need to jump through therings in the following order (1 is close to the entrance and 7 is near the Prophet of Truth): 1, 3, 4, 6, 7, 5, 4, 6, 5, 4, 5, 3, 4. Note that each ring hums a different note- the correct sequence plays out the Halo theme. If done correctly, all the rings will start flashing and you will hear the tune. The Skull will appear on the bridge to the Prophet of Truth. This can be done before or after the scene with the Prophet, but make sure you've cleared out all the enemies. Effect: Alternate dialogue for Campaign mode.

12. Tilt Skull: Level: Cortana

The Skull is in the room with the central glass platform, where Cortana says "Very Nice To Meet You. You Like Games...So Do I". When you enter the room, you'll see some small platforms jutting out of the wall on the left. Jump onto the very tip of the purple mechanism with the glowing blue simbol; it helps to look down so you can see where your feet are. Jump to the left onto the platform. Step out to the very edge and jump to the next higher platform above. Walk to the edge again and jump to the top platform. Walk across the tentacle-like strand to the Skull. - Effect: Enemies have stronger armor and shields.

13. Mythic Skull: Level: Halo

The last one is the easiest. At the start of the level, when you enter the cave, you'll find a crevice in the right wall. Follow it to the skull. - Effect: All enemies have double health.