Where are they

Hint 1 of 4:

You get one for successfully completing an ordeal.

Hint 2 of 4:

Ordeals are rooms that you enter from THE MASTER'S ORBIT.

Hint 3 of 4:

There's 1 skull per door on the outer wall of THE MASTER'S ORBIT.

Hint 4 of 4:

You might also be able to attack another human to get one.