Mission 1 - IWHBYD Skull:
During Mission 1: Osiris, continue through the mission until you reach the doors to the Forerunner security station. Hop up onto the ledge just left of the doorway. Move further along the ledge to find the IWHBYD Skull near the edge of the cliff. This Skull allows uncommon dialogue to become more common, and common dialogue will be heard less.
Mission 2 - Black Eye Skull:
Play through Mission 2: Blue Team until you reach the Reactor Control Room. From the entrance, move to the left and walk up a path on the left side of the room. Climb on top of the square unit partway up the path, then climb onto the large pipe that lines the left wall. Follow along the pipe, and it will lead you toward a higher ledge at the back of the room. Jump to the ledge, and enter the small opening on the left. There is a metal shutter on the floor in the corner. Use your Ground Pound ability to smash through the metal shutter and drop down into an orange-colored side room, where you will find the Skull in the left corner. With the Black Eye Skull enabled, players must melee enemies to regenerate shields, as shields will no longer regenerate automatically.
Mission 3 - Iron Skull:
To find this Skull, you must play on Legendary difficulty during Mission 3: Glassed. Get into a Scorpion parked in one of the garages just before heading uphill to the station gate in the last half of the mission. Drive the Scorpion along the uphill path until you reach the Meridian Station Gate. Use your Scorpion to shoot down the Phaeton in the sky that appears to be chasing a cargo ship. If you miss the Phaeton, reload from the last checkpoint and try again.
Once you successfully shoot down the Phaeton, the Iron Skull will spawn in one of three locations in the garage area. The first location is in between two yellow containers along a fence near a cliffside. The next is found between several containers covered by a green tarp in the garage area. There’s a small staircase leading to these containers just above one of the garages. The last location is near the center of the garage area. There’s a small rocky overhang with two moveable crates underneath. The skull may be found behind these crates. The Iron Skull forces players to restart at the beginning of the mission rather than the last checkpoint, so use this Skull at your own risk.
Mission 5 - Blind Skull:
Continue through Mission 5: Unconfirmed until you reach the passage that leads from the quarry to the indoor mine. Hop up on a nearby crate and jump to a platform between the pipes that run along the ceiling. Follow the walkway as it leads to a small crawlspace in the rock wall to locate the Skull. The Blind Skull removes all aspects of the Head-Up Display (HUD), while also removing the appearance of your hands & weapons, causing players to play without knowing which gun is equipped or how much ammo is left.
Mission 6 - Thunderstorm Skull:
This Skull requires quick maneuvering. Once you begin Mission 6: Evacuation, you must destroy five traffic cones in this starting area in under two minutes to retrieve the Skull at the end of the mission. Begin by getting onto a Mongoose near the starting point, as this will help you quickly move from one cone to the next.
The first cone is found along the left path just after you pass the first gate. Shoot it with your vehicle, and move on. Continue following the main path down the hill until you reach the garage area. The next cone is found near the base of a staircase off to the right. Drive through the passage ahead, and locate another traffic cone on the broken bridge behind some debris on the right. Keep moving forward until you reach a large vehicle with six wheels. Pass behind this vehicle and look for the next cone at the base of a yellow cylindrical unit to the right. Now drive along the left side of the path to find the final cone near the base of some burning debris.
Now, continue through the mission until you reach the part where you must quickly run to the Pelican. As you pass through a hallway leading outside to the landing pad, keep an eye on the ground. If you destroyed all five cones in time, you’ll find the Skull embedded in the ground along this path. The Thunderstorm Skull causes all Campaign enemies to upgrade to their highest ranked versions, for an added challenge.
Mission 7 - Grunt Party Skull:
There are actually two Skulls that can be found during Mission 7: Reunion. The first Skull can be found fairly early on. When you reach an area in the canyon where a group of Covenant appear to be plotting something. Head to the right, and pass through a narrow cave leading to a drop off. You will discover a solitary Grunt sitting at the edge of the drop off, singing a tune while holding the Skull in his hand. Kick the Grunt off the edge, and the Grunt Party Skull is yours to collect. This Skull causes all Grunts killed by headshots to burst into confetti, while Grunts killed without headshots explode like a grenade, dealing damage to anything nearby.
Mission 7 - Cloud Skull:
This Skull is found near the end of Mission 7: Reunion. Continue until you reach the open area leading to the Gateway. As you approach this open area, move up the path to the right. Search a short Forerunner platform structure ahead to find the Cloud Skull. This Skull disables the motion sensor on the HUD, preventing players from detecting nearby enemies.
Mission 8 - Mythic Skull:
This Skull is found during MIssion 8: Swords of Sanghelios. Play through the mission until you pass through the ruins beyond the sinkhole. Once you head up the stairs leading outside, turn right and look for a broken pillar that you can jump up on. From here, you must platform along the other broken pillars leading to the left. The Mythic Skull is found on the largest platform on the far left. When you are standing on the pillar closest to this final platform, sprint and use your thruster to pass over the gap. You can also jump and initiate a Ground Pound about halfway through to reach the platform. This Skull doubles the health of all enemies, increasing their sustain.
Mission 10 - Catch Skull:
To retrieve this Skull, play through Mission 10: Enemy Lines until you are required to fly a Phaeton onto the Kraken. Once you are on the Kraken, head down through the passages until you reach the Banshee hangar. Stand near the Banshees and turn around. There is a platform above the green tunnel on the right. The Skull is found on top this platform near the back wall. You can either fly a Banshee or carefully jump up the lower wall to reach this platform. The Catch Skull increases the amount of grenades that enemies carry and throw.
Mission 12 - Cowbell Skull:
The Cowbell Skull can be found at the very end of Mission 12: Battle of Sunion. Run to the back platform where the Warden spawns, closest to the Guardian. Carefully look over the edge, and drop down onto a metal beam protruding from below. You will find the Skull embedded in the nearby debris. The Cowbell Skull triples the explosive power of all explosive weapons.
Mission 13 - Tilt Skull:
This Skull is found on Genesis near the bridge where the Warden first appears. After the bridge, head up the path to the right. Jump and climb up the rocky structure along the side of the path. You’ll find the Skull near the edge of the cliff. The Tilt Skull generally reduces the effectiveness of weapons depending on the favorability of the target certain weapons are used on.
Mission 14 - Famine Skull:
Continue about halfway through Mission 14: The Breaking. Once the Warden appears for the first time during the mission, head towards the control terminal underneath the platform where the Warden spawned. Now turn around and head into another passage just behind that terminal. The Skull is found on the ground near the center of the passage. The Famine Skull causes weapons that are dropped by enemies and fallen allies to have less ammo.
Mission 15 - Tough Luck:
This Skull is found during Mission 15: Guardians. Once you reach the open area where you are told to destroy the cores, head to the right toward the location of a crashed Pelican. The Skull is found on a high ledge to the left of the Pelican debris, near Gamma Core. Climb up a lower ledge nearby, and scale the icy steps along the wall. Run along the ice ledge and locate the Skull at the base of a pile of rocks to the left of the Pelican wreckage. The Tough Luck Skull causes enemies to be more alert and agile.