Halo 5: Guardians

There are a total of 13 skulls available in Halo 5: Guardians, and only Mission 3 requires Legendary difficulty. They can be seen as follows:
[0:07] - Mission 1 - IWHBYD
[0:40] - Mission 2 - Black Eye
[1:34] - Mission 3 - Iron (Legendary)
[3:33] - Mission 5 - Blind
[4:14] - Mission 6 - Thunderstorm
[6:37] - Mission 7 - Grunt Birthday Party
[7:46] - Mission 7 - Fog
[8:25] - Mission 8 - Mythic
[9:48] - Mission 10 - Catch
[10:42] - Mission 12 - Cowbell
[11:25] - Mission 13 - Tilt
[12:04] - Mission 14 - Famine
[12:30] - Mission 15 - Tough Luck

Upon collecting your first skull you will unlock:

This guide to collecting all the skulls will include descriptions for each of them within the levels along with a video guide for every individual skull. Plan to add an all skulls video as well afterwards. Hope you appreciate the written portions to complement the videos and also want to mention Normal is needed to collect skulls along with Legendary for one particular skull which will be noted.

Mission 1: Osiris "Skull: IWHBYD"
Move across the level normally until you reach the point where you need to smash through the ice. The next portion is where the first and easiest skull in the game is located. Run through this open area where there's tons of enemies or kill them if needed on at least Normal difficulty. The skull will be to the left of the exit to the next part of the level and you'll need to climb on a mini cliff side. On the portion of the mini cliff side furthest towards the open outside of map battle you'll find the tiny IWHBYD skull sitting there.


Mission 2: Blue Team "Skull: Black Eye"
With this skull there are two ways to go about collecting it. This skull is found at the far back of the reactor room where you need to flick a switch in the far back. It can be quickly collected by backing up just before the reactor in the middle of the room and climbing the left side on top of a pipe. Continuing along there you'll reach a dark portion of the level with a vent, do the ground smash and it'll take you to this dark red room. At the far back on the wall opposite the ammo stack there's a canister with the skull on it. The other option is clearing out enemies by the switch and the door to the room will open eventually.


Mission 3: Glassed "Skull: Iron"
This is honestly the most difficult skull in the game since it needs to be completed on Legendary difficulty. You will need the scorpion which can be found by breaking into the vents at the base just below the steep ramps. Be cautious of the many turrets that are lining the way, though they can shot at a distance. From there you move onwards up the ramps and then turn right to continue the level as you usually would. Just before climbing the next hill you'll see a human carrier ship being chased by a Promethean ship which is orange and gray. You need to destroy that using the turret on the scorpion since it's easiest. If the carrier ship gets destroyed or goes off-screen simply restart the checkpoint. The grunt birthday party sound will play if you did it correctly. The next part is hard to explain since it the skull could be in three areas. The first is straight ahead on some rocks below the event by a fence, the next is back on top of where you got the scorpion behind a canister and the third is underneath some rocks by block crates which is just to the side of the scorpion location. Video shows the three spots, may add pictures if asked.


Mission 5: Unconfirmed "Skull: Blind"
This skull can be found just after finishing the portion where you need to scan the prowler vehicle that Blue Team was in. Once you enter through that door that takes awhile to open since they're talking head left. There will be some items to climb and then you'll see a panel to climb onto the roof with, follow along until you see a vent. Crouch into this vent and the skull is at the end!


Mission 6: Evacuation "Skull: Thunderstorm"
This is actually another difficult one to collect. There are five pylons you need to destroy within two minutes in order to collect the skull. I'll try my best to explain though I think the video displays it better, may add photos if requested. Grab the mongoose with rockets at the start if solo or the warthog if in a group, you can't miss these. Additionally you will hear the grunt birthday party sound upon hitting each.

Pylon 1: Cut right once entering the first open area and drive along until you see some lengthy crates, the pylon is sitting there.
Pylon 2: Continue along and jump down the flat area which leads into the lower next big space. Move to the right just before the bridge and pylon will be at the based of the steps, sometimes enemies will take this out for you so listen for the sound.
Pylon 3: This is located on the bridge towards the end on the other side of a smoldering warthog, also watch out for a Promethean warthog since it may cause issues at this portion.
Pylon 4: Head straight ahead and move towards the right where there's a fork and then head to the center once more. The pylon is front of this moving contraption.
Pylon 5: Continue along the map and you'll see the crashed smoldering pelican, pylon will be underneath. If it isn't there you need to restart the level.

After completing this and getting all give you need to finish the level by any means. Right before the end run to the pelican the skull will be in the floor, hard to miss.



Mission 7: Reunion "Skull: Grunt Birthday Party" (1/2)
This level has two skulls on it and the first is Grunt Birthday Party. At a certain point not too into the game you'll reach this strangely open and empty area. Huge the right and there will be this long out of place tunnel. At the end is a grunt talking to himself, click whatever the punt prompt is to kick the poor guy and pick up his prize skull.


Mission 7: Reunion "Skull: Cloud" (2/2)
This is the second skull in what I would call the back half of the level. This is literally found right at the end before you towards the massive Forerunner structure just cut right to this open area. This area splits apart so hug the left portion and the skull will just be sitting there in the open on a platform.


Mission 8: Swords of Sangelios "Skull: Mythic"
This is a slightly difficult skull to obtain since it involves some climbing. About halfway through the level you will come across this more open space where you have to jump a gap. Before that you leave this structure area with huge statues and there's a low fallen pillar you walk under. You need to climb this using the jet feature and also climbing up. Once you reach the tip top of that you can look out into the bright open to see a platform. You will need to spring jump and then jet across the climb onto the edge. The skull will be sitting there.


Mission 10: Enemy Lines "Skull: Catch"
This skull is found on the kraken which the giant purple covenant vehicle you see through the level and basically at the end of the mission in general. Once on board you make your way down until you reach a bay with two banshees. There will be a ledge just beside them which can be climbed or flown onto with the banshee. There's two sides to this thing since it's essentially a cylinder in design so if not in one area, check the other side.



Mission 12: Battle of Sunaion "Skull: Cowbell"
This is found right at the end of the level furthest towards the Guardian floating above. Hopefully you can run by and grab before the mini boss fight that takes place. You will run all the way towards the end of the level and look down. There will be a piece of metal sticking out, just fall down into this small area and the skull is nestled there, really easy!


Mission 13: Genesis "Skull: Tilt"
This is found fairly early in the level after using the scorpion you will come across this bridge that needs to be activated by the local monitor. Once it's open you'll cross and see the Warden, destroy the turrets. Then you can head right up this path and there will be rocks before you head up a grass path. Climb these various levels which can be slightly difficult and the skull can be found at the peak of this area.


Mission 14: The Breaking "Skull: Famine"
After making it halfway through the level there will be a giant flash of light ball after going through a door. Just before that you destroy two turrets for better reference. The Warden is found in this light that appears, cut left and head behind him, the skull is just sitting there out in the open.


Mission 15: Guardians "Skull: Tough Luck"
This is the final skull found in the large open area where you need to destroy spheres. The skull can be found to the right of the more open portion along a snowy cliff which has a crashed pelican. Climb over the pelican and the skull can be found nestled with some rocks just below.


Hope this guide helps everyone, full video to be added later and if more descriptions are needed for any let me know!

I wanted to include all intel AND skulls into one easy-to-use video for those that are interested in that sort of thing. Rather than timestamp each individual item, I've included timestamps of each mission's starting point below. If enough folks express interest, I can copy and paste the written directions to each one into this solution (roughly a paragraph or two for each mission, directing you on where they all are) from the Walkthrough, although it should be published today.


Mission 1 starts at 00:07
Mission 2 starts at 02:30
Mission 3 starts at 05:09
Mission 4 starts at 08:57
Mission 5 starts at 10:51
Mission 6 starts at 13:19
Mission 7 starts at 18:17
Mission 8 starts at 21:21
Mission 9 starts at 25:41
Mission 10 starts at 28:13
Mission 11 starts at 31:29
Mission 12 starts at 33:11
Mission 13 starts at 35:56
Mission 14 starts at 39:00
Mission 15 starts at 40:06

There are a total of 117 intel to collect, and 13 skulls to collect in the game. You can see how many you are missing in each mission by looking in the pause menu and selecting the option for mission intel. This will tell you the names of the ones you have found. Skulls can be checked in the same way. You can check every mission by looking at your service record, which can be found on the main menu.
None of the collectibles are missable. You can go back and replay missions to pick up any you may have missed. If you happen to die after picking up an intel, or claiming a skull, it will save and you won't need to pick it up again. You will also get 4 achievements for collecting all the intel and skulls. Those being; Gravedigger, Conspiracy Theory, Hunt the Truth and Gravelord.

https://www.youtube.com/playlist?list=PLSz08HX7PxlU9Obr8LOoR...