Mission 1: Osiris - IWHBYD Skull “Rare combat dialogue becomes more common.”
Mission 2: Blue Team - Black Eye Skull “Shields don’t recharge unless you melee enemies.”
Mission 3: Alliance - Iron Skull “Cannot revive or be revived. Solo: Restart mission on death. Co-op: Spawn at last checkpoint on death.”
The next step is to look for the Skull. It will be in one of three places: 1) Hiding under a yellow barrel on top of the Scorpion hangar, 2) In between two yellow barrels on the left fence, right before you reach the building on the left when the Phaeton appeared, 3) Near two metal boxes under one of the two small archways in the open battlefield you fought in before picking up the Scorpion.
Mission 5: Unconfirmed - Blind Skull “The HUD and the first person arms and weapon are hidden.”
Mission 6: Evacuation - Thunderstorm Skull “Major upgrade to the capabilities of enemies.”
Aside from Mission 3, this is the only other time-sensitive Skull quest in Halo 5. The first step is to fire at (and destroy) five traffic cones. Take a Mongoose at the beginning of the mission and immediately veer left toward the mining station nearby. Note the traffic cone next to the rock wall and near a couple boxes.
For the second cone, tear down the hill until you’re back at the battleground where you picked up the Scorpion. Right before you reach the bridge entrance, look to your right and notice the cone next to the mining station. Then as you cross the bridge, note the brunt-out Warthog chassis to your right; a traffic cone will be there. You’ll soon find yourself ascending up a hill and when that happens, keep your eyes on the right. The cone will be found next to a yellow column on one of the nearby small generators.
The final cone is by the wreckage of a UNSC vehicle; if you’re going up the hill, it will be on your left. Assuming you reach this wreckage within 2 minutes (from the first cone you destroy), you’ll see the final traffic cone near to that vehicle. When you’ve destroyed the last cone, progress through the mission until you’re rushing to the Pelican for your escape. Look for the Skull on the ground along the path when you’re on the same level as the Pelican.
Mission 7: Reunion - Grunt Birthday Party Skull “Headshot a Grunt - HAPPY BOOM TIME!”
This Skull is found near the crashed ship surrounded by Covenant near the beginning of the mission. As you make your way to the ship, head toward the ledges on your right, but before you ascend up the ledges note the fork in the path; take the newly-discovered path on the right. Kill the carefree Grunt in order to get the Skull.