Below is a list of the Skulls and where you can find them in Halo Infinite.
1. Boom
This skull can be found during the Warship Gbraakon mission. Towards the end of the ship, you’ll come to a long room with two levels and three moving platforms on the right side transporting metal crates. You’ll want to head to the upper level then hop on the middle platform to ride it up. The skull will be resting on an adjacent crate.
2. Cowbell
This skull can be found during the Foundation mission. Proceed through the level until you collect the Weapon then keep moving forward until you enter a hallway and it talks about something behind ahead and calling out to it. Exit the hallway and then use your status button to locate the skull up in a rafter over the gap in the center of the room. You’ll need to use your Grappleshot to reach it.
3. Mythic
This skull can be found during The Command Spire mission. As you proceed through the mission, you’ll come across pillars that emerge vertically from a yellow energy barrier. From this point, keep an eye out for the pillars that are moving horizontally. Head past 2 walls of the yellow barriers to reach the end of the room where the pillars will turn right and be sent upward. In the room where they turn right, there will be an open square section in the ceiling. Grapple up there to find a door that leads to the room containing the skull.
4. Grunt Birthday Party
This skull can be found during the Repository mission. Proceed through the level until you receive Escharum’s message via cutscene. There will then be a bridge linking you to a room straight ahead.
Facing the doors to that room, look to the right of the terminal to find an area you can grapple to with another door. Grapple to that balcony and head through the doorway to find a power seed at the end. Return to the room with the bridge then grapple to the room across the way. Enter the door and on the right side of the room you’ll find a place you can put the power seed. Doing this deactivates a barrier further in the level that allows you access to the skull. Keep heading through the level until you come to the room shown below.
Grapple on top of the structure’s support beams to find the skull. Beware of the 4 invisible Elites that guard this skull. You can use the Threat Sensor to locate them.
5. Bandana
This skull can be found during the Silent Auditorium mission. Before hunting down this skull, it is important to note that you must not do any damage to the Sentinels. Doing so will make the skull inaccessible. Work your way through the level, dealing with the Banished, until you meet up with the floating sphere. Shortly after meeting him, you’ll come to an area where you have to activate a light bridge. Pressing the switch will activate 2 light bridges (see above). Head straight across to reach a blue lit door. The skull will be located in the room behind this door!
6. Thunderstorm
This skull is located at the top of a hexagon column west of the northern Beacon in the southeastern region of Zeta Halo. You’ll need a Wasp to reach it.
7. Fog
This skull can be found on a pile of helmets in a nook directly west of FOB Alpha along the map’s edge.
8. Blind
This skull can be found on a platform in the chasm southwest of Carrera Squad.
9. IWHBYD
This skull can be found on the roof of The Tower. It is reachable by climbing up the outside of The Tower using the Grappleshot (with the Quickshot upgrade).
10. Famine
This skull can be found on the island south of the eastern Beacon in the southeastern region of the map.
11. Black Eye
This skull is located in the middle of a waterfall southeast of FOB Juliet in the southeastern region of the map. You’ll have to grabble to a small hole in the middle of the waterfall to reach the cave containing the skull.
12. Catch
This skull can be found in a mound of skulls in the northern portion of the northwestern region next to 2 red Spartan Hunters. You can access this area via Grappleshot-ing to the island or flying the Wasp (if you’ve unlocked it with Valor points).