A ghost Skull is a non-fighting spirit found in the Mars city Underground Systems during the Invasion. When the gateway was opened, Ghost Skulls got from the teleporter and transformed nearly all of the base personnel into Zombies. The Ghost Skulls had come along with the initial Shockwave that passed through the entire base.
The Purpose of the Ghost Skull was originally to transform the Humans in Mars City into Zombies and adding them,the forces of hell. But another porpuse was to transform the marines and make them as a type of Super Warriors called Commandos. With that the Hellish Swarms became stronger, and used the Human weapons and technology against them.