Collect all 30 Crystal Skulls to unlock the Giant Crystal Skull. At the 
beginning of Chapter 13, after going through a few rooms with enemies, you will 
reach a large green pillar in a room with centaurs. The Giant Crystal Skull is 
also there. Its description reads "Obtainable only by those with superhuman 
powers of observation." Its detailed description reads "A gigantic crystal skull 
from ancient legend. According to legend, this enormous skull belonged to a 
giant who walked the earth long ago. In these ancient times, the world knew no 
peace, and tribes fought each other to near extinction. None could see an end to 
this chain of suffering, until the giant appeared. He vowed to bear the sins of 
mankind upon his own soul, and offered himself as a sacrifice to the gods. His 
skull was then locked away in a room overflowing with human suffering." The 
legend ends with the warning, "He who possesses this skull must accept the date 
of the giant as his own."