Grab a banshee and fly over to it, get out of the banshee, and collect the skull that is in plain sight. Once in the correct bay, look up to the left to see a small platform in this room. If it is to your left, it is correct if it is to your right, switch to the other bay. You will need to find the correct bay to do so, look at the back of the banshees and find the greenlit hall. Mission 10 - Catch: Once on the Kraken, go down and find one of the two Banshee docking bays. The skull is in plain sight once on the platform. To do this, jump on the diagonal beam in the doorway of the entrance to this area and then sprint, jump, dash, and mantle your way up. Mission 8 - Mythic: After you walk through the ruins and see the covenant reinforcements, you will need to navigate to a high platform on your left as you enter the area. Do not go here yet since it ends the level instead, go to the right and follow the path up the hill to find the skull on the platform. Mission 7 - Fog: At the end of the level, after exiting the underground cave system that you can walk or fly through, notice a large structure in front of you which is where your objective is. Click the "Punt" prompt, and the skull will replace him after he flies off. Follow the path to the end to find the singing grunt holding a skull. Mission 7 - Grunt Birthday Party: After your first enemy encounter near the downed covenant ship, get on the path above and behind it. Upon destroying all five traffic cones (you will hear the Grunt Birthday Party sound effect after destroying each cone), proceed to the end of the level to find the skull on the ground as you enter outside as the Guardian begins to "pulse." Jump into a Gungoose if you are alone, or Warthog if you have a partner that can drive/take the turret in order to do this quickly. The final traffic cone will unspawn if you are not fast enough. Mission 6 - Thunderstorm: This skull requires you to destroy five traffic cones in two minutes at the very start of the level. Then, follow the vents to find a small cave/path with the skull. Once it opens, get on the left ledge, then jump on the box and jump onto the vents at the top of the room. Mission 5 - Blind: After the sniper section, go to the top of the hill and the large vault door will open. Collect the skull from one of these three locations to unlock it. After taking down the Phaeton, the skull will spawn in one of three locations - either alongside the fence in the nearby area in between two yellow barrels (this is just before the tower you passed), on top of the roof of the Scorpion bay underneath a yellow barrel, or under the small archway near the entrance of the area close to the bridge next to two metal boxes. If successful, you will hear the Grunt Birthday Party sound effect otherwise, reload the last checkpoint. You can do this using the Scorpion's main gun, or turret, or a combination of both in co-op. You must shoot down the Phaeton before it takes out the cargoship. A Phaeton will begin the chase it and shoot at it.
As you move past the big building to the left, you will see a friendly cargo ship appear in the sky in front of you. You go under a large rock arch way and follow the path to the right. After getting into the Scorpion, proceed up the hill and take out the beam turrets. If you try on any other difficulty, you will not be able to shoot down the Phaeton. Mission 3 - Iron (Legendary): This skull can only be attained on the Legendary difficulty. Ground Pound through the vent in the floor into a red room where you will find the skull on a table. Follow the pipes to the back of the room and jump across to the ledge when they end. Use the cache to get on top of the large pipes on the side of the wall. Mission 2 - Black Eye: Once you reach the reactor room, clear out a few enemies, then go the very left side of the room near the ammo cache. Mission 1 - IWHBYD (I Would Have Been Your Daddy): After the second large fight area, before entering the room with the elevator, jump up the the left side cliff to find the skull at the end of this ledge. Search the indicated locations to find all 14 Skulls and get the "Gravelord" achievement. Nightfall helmet: Watch Halo: Nightfall - Episode 1, "Second Story".
Nightfall armor: Watch all five episodes of Halo: Nightfall. Helioskrill armor and helmet: Unlock the "Legend" achievement in Halo: The Master Chief Collection. Mark VI (Gen 1) Scarred armor and helmet: Unlock the "Cairo Stations" achievement in Halo: The Master Chief Collection. Mark VI (Gen 1) armor and helmet: Unlock the "Your Journey Begins" achievement in Halo: The Master Chief Collection. Successfully complete the indicated task to unlock the corresponding armor and/or helmet: