This skull list is made by Ageha:
Part 1: Holdenfort
1. Enter the fort on the left side by jumping over the fence. Run a bit forward and you can see it.
2. Now go back out and run to the right entrance. Run down the docks as you can see on the pictures.
3. Continue down the docks and cross the stream by jumping on the floodgate.
Follow the way on the other side. The skull is on the left side behind crates.
4. From skull 3 go back and follow the way up the ramp. Follow the way on the pictures.
Jump up the crate and you see the skull down by the house.
5. Now go back and up the big stairs. It’s behind the fence on the right side.
6. Now go back where you found skull 4 and continue the way up.
You can cross the bridge here to avoid fighting Legions.
Do a few jumps to come across the bridge.
First hang down by pressing B. Then look in the direction you want to jump and press Y.
Follow the way on the others side and you can see the last skull.
Part 2: Caldberg
1. When you enter the fort on the right side by climbing up you can already see a skull in the background.
2. Now go back up where you climbed in and run left. Run along the left wall to avoid to be seen from the Legions down on the right. On the far end climb down the net and get the skull by the hut.
3. Now back up the net and down the wooden construction. After the first ladder turn left, down again and to the passageway. Now down a net to the walkway. Turn right and the skull is behind a crate.
4. Follow the walkway until you reach the dead end. Climb up the net, turn right and you see the next skull.
5. Run back from where you climbed up and run straight and a bit right to find a Leystone. From there run up the stairs, a bit right and into an alley. The skull is at the dead end.
6. Run back and right up the stairs. One more stair up then turn right to find the skull.